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.

src/include/nodes/parsenodes.h

@@ -168,8 +168,7 @@ typedef struct Query
  * specify the type by OID than by name.  If "names" is NIL then the
  * actual type OID is given by typeOid, otherwise typeOid is unused.
  * Similarly, if "typmods" is NIL then the actual typmod is expected to
- * be prespecified in typemod, otherwise typemod is unused.  Similarly
- * for collnames/collOid.
+ * be prespecified in typemod, otherwise typemod is unused.
  *
  * If pct_type is TRUE, then names is actually a field name and we look up
  * the type of that field.	Otherwise (the normal case), names is a type
@@ -185,8 +184,6 @@ typedef struct TypeName
 	List	   *typmods;		/* type modifier expression(s) */
 	int32		typemod;		/* prespecified type modifier */
 	List	   *arrayBounds;	/* array bounds */
-	List	   *collnames;		/* collation name */
-	Oid			collOid;		/* collation by OID */
 	int			location;		/* token location, or -1 if unknown */
 } TypeName;
 
@@ -468,6 +465,10 @@ typedef struct RangeFunction
  * how this ColumnDef node was created (by parsing, or by inheritance
  * from an existing relation).	We should never have both in the same node!
  *
+ * Similarly, we may have a COLLATE specification in either raw form
+ * (represented as a CollateClause with arg==NULL) or cooked form
+ * (the collation's OID).
+ *
  * The constraints list may contain a CONSTR_DEFAULT item in a raw
  * parsetree produced by gram.y, but transformCreateStmt will remove
  * the item and set raw_default instead.  CONSTR_DEFAULT items
@@ -485,6 +486,8 @@ typedef struct ColumnDef
 	char		storage;		/* attstorage setting, or 0 for default */
 	Node	   *raw_default;	/* default value (untransformed parse tree) */
 	Node	   *cooked_default; /* default value (transformed expr tree) */
+	CollateClause *collClause;	/* untransformed COLLATE spec, if any */
+	Oid			collOid;		/* collation OID (InvalidOid if not set) */
 	List	   *constraints;	/* other constraints on column */
 } ColumnDef;
 
@@ -1202,9 +1205,8 @@ typedef struct AlterTableCmd	/* one subcommand of an ALTER TABLE */
 	AlterTableType subtype;		/* Type of table alteration to apply */
 	char	   *name;			/* column, constraint, or trigger to act on,
 								 * or new owner or tablespace */
-	Node	   *def;			/* definition of new column, column type,
-								 * index, constraint, or parent table */
-	Node	   *transform;		/* transformation expr for ALTER TYPE */
+	Node	   *def;			/* definition of new column, index,
+								 * constraint, or parent table */
 	DropBehavior behavior;		/* RESTRICT or CASCADE for DROP cases */
 	bool		missing_ok;		/* skip error if missing? */
 	bool		validated;
@@ -1819,6 +1821,7 @@ typedef struct CreateDomainStmt
 	NodeTag		type;
 	List	   *domainname;		/* qualified name (list of Value strings) */
 	TypeName   *typeName;		/* the base type */
+	CollateClause *collClause;	/* untransformed COLLATE spec, if any */
 	List	   *constraints;	/* constraints (list of Constraint nodes) */
 } CreateDomainStmt;