Special case C_COLLATION_OID in pg_newlocale_from_collation().

Allow pg_newlocale_from_collation(C_COLLATION_OID) to work even if
there's no catalog access, which some extensions expect.

Not known to be a bug without extensions involved, but backport to 18.

Also corrects an issue in master with dummy_c_locale (introduced in
commit 5a38104b36) where deterministic was not set. That wasn't a bug,
but could have been if that structure was used more widely.

Reported-by: Alexander Kukushkin <cyberdemn@gmail.com>
Reviewed-by: Alexander Kukushkin <cyberdemn@gmail.com>
Discussion: https://postgr.es/m/CAFh8B=nj966ECv5vi_u3RYij12v0j-7NPZCXLYzNwOQp9AcPWQ@mail.gmail.com
Backpatch-through: 18

src/backend/utils/adt/pg_locale.c

@@ -134,6 +134,13 @@ static pg_locale_t default_locale = NULL;
 static bool CurrentLocaleConvValid = false;
 static bool CurrentLCTimeValid = false;
 
+static struct pg_locale_struct c_locale = {
+	.provider = COLLPROVIDER_LIBC,
+	.deterministic = true,
+	.collate_is_c = true,
+	.ctype_is_c = true,
+};
+
 /* Cache for collation-related knowledge */
 
 typedef struct
@@ -1194,6 +1201,13 @@ pg_newlocale_from_collation(Oid collid)
 	if (collid == DEFAULT_COLLATION_OID)
 		return default_locale;
 
+	/*
+	 * Some callers expect C_COLLATION_OID to succeed even without catalog
+	 * access.
+	 */
+	if (collid == C_COLLATION_OID)
+		return &c_locale;
+
 	if (!OidIsValid(collid))
 		elog(ERROR, "cache lookup failed for collation %u", collid);