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/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>