Remove collation information from TypeName, where it does not belong.

The initial collations patch treated a COLLATE spec as part of a TypeName,
following what can only be described as brain fade on the part of the SQL
committee.  It's a lot more reasonable to treat COLLATE as a syntactically
separate object, so that it can be added in only the productions where it
actually belongs, rather than needing to reject it in a boatload of places
where it doesn't belong (something the original patch mostly failed to do).
In addition this change lets us meet the spec's requirement to allow
COLLATE anywhere in the clauses of a ColumnDef, and it avoids unfriendly
behavior for constructs such as "foo::type COLLATE collation".

To do this, pull collation information out of TypeName and put it in
ColumnDef instead, thus reverting most of the collation-related changes in
parse_type.c's API.  I made one additional structural change, which was to
use a ColumnDef as an intermediate node in AT_AlterColumnType AlterTableCmd
nodes.  This provides enough room to get rid of the "transform" wart in
AlterTableCmd too, since the ColumnDef can carry the USING expression
easily enough.

Also fix some other minor bugs that have crept in in the same areas,
like failure to copy recently-added fields of ColumnDef in copyfuncs.c.

While at it, document the formerly secret ability to specify a collation
in ALTER TABLE ALTER COLUMN TYPE, ALTER TYPE ADD ATTRIBUTE, and
ALTER TYPE ALTER ATTRIBUTE TYPE; and correct some misstatements about
what the default collation selection will be when COLLATE is omitted.

BTW, the three-parameter form of format_type() should go away too,
since it just contributes to the confusion in this area; but I'll do
that in a separate patch.

doc/src/sgml/ref/alter_table.sgml

@@ -32,9 +32,9 @@ ALTER TABLE <replaceable class="PARAMETER">name</replaceable>
 
 <phrase>where <replaceable class="PARAMETER">action</replaceable> is one of:</phrase>
 
-    ADD [ COLUMN ] <replaceable class="PARAMETER">column</replaceable> <replaceable class="PARAMETER">data_type</replaceable> [ <replaceable class="PARAMETER">column_constraint</replaceable> [ ... ] ]
+    ADD [ COLUMN ] <replaceable class="PARAMETER">column</replaceable> <replaceable class="PARAMETER">data_type</replaceable> [ COLLATE <replaceable class="PARAMETER">collation</replaceable> ] [ <replaceable class="PARAMETER">column_constraint</replaceable> [ ... ] ]
     DROP [ COLUMN ] [ IF EXISTS ] <replaceable class="PARAMETER">column</replaceable> [ RESTRICT | CASCADE ]
-    ALTER [ COLUMN ] <replaceable class="PARAMETER">column</replaceable> [ SET DATA ] TYPE <replaceable class="PARAMETER">type</replaceable> [ USING <replaceable class="PARAMETER">expression</replaceable> ]
+    ALTER [ COLUMN ] <replaceable class="PARAMETER">column</replaceable> [ SET DATA ] TYPE <replaceable class="PARAMETER">data_type</replaceable> [ COLLATE <replaceable class="PARAMETER">collation</replaceable> ] [ USING <replaceable class="PARAMETER">expression</replaceable> ]
     ALTER [ COLUMN ] <replaceable class="PARAMETER">column</replaceable> SET DEFAULT <replaceable class="PARAMETER">expression</replaceable>
     ALTER [ COLUMN ] <replaceable class="PARAMETER">column</replaceable> DROP DEFAULT
     ALTER [ COLUMN ] <replaceable class="PARAMETER">column</replaceable> { SET | DROP } NOT NULL
@@ -115,7 +115,11 @@ ALTER TABLE <replaceable class="PARAMETER">name</replaceable>
       This form changes the type of a column of a table. Indexes and
       simple table constraints involving the column will be automatically
       converted to use the new column type by reparsing the originally
-      supplied expression.  The optional <literal>USING</literal>
+      supplied expression.
+      The optional <literal>COLLATE</literal> clause specifies a collation
+      for the new column; if omitted, the collation is the default for the
+      new column type.
+      The optional <literal>USING</literal>
       clause specifies how to compute the new column value from the old;
       if omitted, the default conversion is the same as an assignment
       cast from old data type to new.  A  <literal>USING</literal>

doc/src/sgml/ref/alter_type.sgml

@@ -32,9 +32,9 @@ ALTER TYPE <replaceable class="PARAMETER">name</replaceable> ADD VALUE <replacea
 
 <phrase>where <replaceable class="PARAMETER">action</replaceable> is one of:</phrase>
 
-    ADD ATTRIBUTE <replaceable class="PARAMETER">attribute_name</replaceable> <replaceable class="PARAMETER">data_type</replaceable> [ CASCADE | RESTRICT ]
+    ADD ATTRIBUTE <replaceable class="PARAMETER">attribute_name</replaceable> <replaceable class="PARAMETER">data_type</replaceable> [ COLLATE <replaceable class="PARAMETER">collation</replaceable> ] [ CASCADE | RESTRICT ]
     DROP ATTRIBUTE [ IF EXISTS ] <replaceable class="PARAMETER">attribute_name</replaceable> [ CASCADE | RESTRICT ]
-    ALTER ATTRIBUTE <replaceable class="PARAMETER">attribute_name</replaceable> [ SET DATA ] TYPE <replaceable class="PARAMETER">data_type</replaceable> [ CASCADE | RESTRICT ]
+    ALTER ATTRIBUTE <replaceable class="PARAMETER">attribute_name</replaceable> [ SET DATA ] TYPE <replaceable class="PARAMETER">data_type</replaceable> [ COLLATE <replaceable class="PARAMETER">collation</replaceable> ] [ CASCADE | RESTRICT ]
 </synopsis>
  </refsynopsisdiv>
 

doc/src/sgml/ref/create_domain.sgml

@@ -89,8 +89,9 @@ CREATE DOMAIN <replaceable class="parameter">name</replaceable> [ AS ] <replacea
       <listitem>
        <para>
         An optional collation for the domain.  If no collation is
-        specified, the database default collation is used (which can
-        be overridden when the domain is used to define a column).
+        specified, the underlying data type's default collation is used.
+        The underlying type must be collatable when <literal>COLLATE</>
+        is specified.
        </para>
       </listitem>
      </varlistentry>

doc/src/sgml/ref/create_table.sgml

@@ -248,9 +248,9 @@ CREATE [ [ GLOBAL | LOCAL ] { TEMPORARY | TEMP } | UNLOGGED ] TABLE [ IF NOT EXI
     <term><literal>COLLATE <replaceable>collation</replaceable></literal></term>
     <listitem>
      <para>
-      The <literal>COLLATE</> clause assigns a nondefault collation to
-      the column.  By default, the locale settings of the database are
-      used.
+      The <literal>COLLATE</> clause assigns a collation to
+      the column (which must be of a collatable data type).
+      If not specified, the column data type's default collation is used.
      </para>
     </listitem>
    </varlistentry>