Don't copy datlocale from template unless provider matches.

During CREATE DATABASE, if changing the locale provider, require that
a new locale is specified rather than trying to reinterpret the
template's locale using the new provider.

This only affects the behavior when the template uses the builtin
provider and CREATE DATABASE specifies the ICU provider without
specifying the locale. Previously, that may have succeeded due to
loose validation by ICU, whereas now that will cause an error. Because
it can cause an error, backport only to unreleased versions.

Discussion: https://postgr.es/m/5038b33a6dc639009f4b3d43fa6ae0c5ba9e04f7.camel@j-davis.com
Backpatch-through: 18

src/backend/commands/dbcommands.c

@@ -1052,7 +1052,7 @@ createdb(ParseState *pstate, const CreatedbStmt *stmt)
 		dbctype = src_ctype;
 	if (dblocprovider == '\0')
 		dblocprovider = src_locprovider;
-	if (dblocale == NULL)
+	if (dblocale == NULL && dblocprovider == src_locprovider)
 		dblocale = src_locale;
 	if (dbicurules == NULL)
 		dbicurules = src_icurules;