Support C.UTF-8 locale in the new builtin collation provider.

The builtin C.UTF-8 locale has similar semantics to the libc locale of
the same name. That is, code point sort order (fast, memcmp-based)
combined with Unicode semantics for character operations such as
pattern matching, regular expressions, and
LOWER()/INITCAP()/UPPER(). The character semantics are based on
Unicode simple case mappings.

The builtin provider's C.UTF-8 offers several important advantages
over libc:

 * faster sorting -- benefits from additional optimizations such as
   abbreviated keys and varstrfastcmp_c
 * faster case conversion, e.g. LOWER(), at least compared with some
   libc implementations
 * available on all platforms with identical semantics, and the
   semantics are stable, testable, and documentable within a given
   Postgres major version

Being based on memcmp, the builtin C.UTF-8 locale does not offer
natural language sort order. But it is an improvement for most use
cases that might otherwise use libc's "C.UTF-8" locale, as well as
many use cases that use libc's "C" locale.

Discussion: https://postgr.es/m/ff4c2f2f9c8fc7ca27c1c24ae37ecaeaeaff6b53.camel%40j-davis.com
Reviewed-by: Daniel Vérité, Peter Eisentraut, Jeremy Schneider

doc/src/sgml/charset.sgml

@@ -377,13 +377,21 @@ initdb --locale-provider=icu --icu-locale=en
      <listitem>
       <para>
        The <literal>builtin</literal> provider uses built-in operations. Only
-       the <literal>C</literal> locale is supported for this provider.
+       the <literal>C</literal> and <literal>C.UTF-8</literal> locales are
+       supported for this provider.
       </para>
       <para>
        The <literal>C</literal> locale behavior is identical to the
        <literal>C</literal> locale in the libc provider. When using this
        locale, the behavior may depend on the database encoding.
       </para>
+      <para>
+       The <literal>C.UTF-8</literal> locale is available only for when the
+       database encoding is <literal>UTF-8</literal>, and the behavior is
+       based on Unicode. The collation uses the code point values only. The
+       regular expression character classes are based on the "POSIX
+       Compatible" semantics, and the case mapping is the "simple" variant.
+      </para>
      </listitem>
     </varlistentry>
 
@@ -878,6 +886,23 @@ SELECT * FROM test1 ORDER BY a || b COLLATE "fr_FR";
       </listitem>
      </varlistentry>
 
+     <varlistentry>
+      <term><literal>pg_c_utf8</literal></term>
+      <listitem>
+       <para>
+        This collation sorts by Unicode code point values rather than natural
+        language order.  For the functions <function>lower</function>,
+        <function>initcap</function>, and <function>upper</function>, it uses
+        Unicode simple case mapping.  For pattern matching (including regular
+        expressions), it uses the POSIX Compatible variant of Unicode <ulink
+        url="https://www.unicode.org/reports/tr18/#Compatibility_Properties">Compatibility
+        Properties</ulink>.  Behavior is efficient and stable within a
+        <productname>Postgres</productname> major version.  This collation is
+        only available for encoding <literal>UTF8</literal>.
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry>
       <term><literal>C</literal> (equivalent to <literal>POSIX</literal>)</term>
       <listitem>

doc/src/sgml/ref/create_collation.sgml

@@ -99,7 +99,7 @@ CREATE COLLATION [ IF NOT EXISTS ] <replaceable>name</replaceable> FROM <replace
       <para>
        If <replaceable>provider</replaceable> is <literal>builtin</literal>,
        then <replaceable>locale</replaceable> must be specified and set to
-       <literal>C</literal>.
+       either <literal>C</literal> or <literal>C.UTF-8</literal>.
       </para>
      </listitem>
     </varlistentry>

doc/src/sgml/ref/create_database.sgml

@@ -166,8 +166,9 @@ CREATE DATABASE <replaceable class="parameter">name</replaceable>
        </para>
        <para>
         If <xref linkend="create-database-locale-provider"/> is
-        <literal>builtin</literal>, then <replaceable>locale</replaceable>
-        must be specified and set to <literal>C</literal>.
+        <literal>builtin</literal>, then <replaceable>locale</replaceable> or
+        <replaceable>builtin_locale</replaceable> must be specified and set to
+        either <literal>C</literal> or <literal>C.UTF-8</literal>.
        </para>
        <tip>
         <para>
@@ -228,9 +229,11 @@ CREATE DATABASE <replaceable class="parameter">name</replaceable>
         linkend="create-database-locale-provider">locale provider</link> must
         be <literal>builtin</literal>.  The default is the setting of <xref
         linkend="create-database-locale"/> if specified; otherwise the same
-        setting as the template database.  Currently, the only available
-        locale for the <literal>builtin</literal> provider is
-        <literal>C</literal>.
+        setting as the template database.
+       </para>
+       <para>
+        The locales available for the <literal>builtin</literal> provider are
+        <literal>C</literal> and <literal>C.UTF-8</literal>.
        </para>
       </listitem>
      </varlistentry>

doc/src/sgml/ref/initdb.sgml

@@ -288,8 +288,9 @@ PostgreSQL documentation
        </para>
        <para>
         If <option>--locale-provider</option> is <literal>builtin</literal>,
-        <option>--locale</option> must be specified and set to
-        <literal>C</literal>.
+        <option>--locale</option> or <option>--builtin-locale</option> must be
+        specified and set to <literal>C</literal> or
+        <literal>C.UTF-8</literal>.
        </para>
       </listitem>
      </varlistentry>