Add the ability for the core grammar to have more than one parse target.

This patch essentially allows gram.y to implement a family of related
syntax trees, rather than necessarily always parsing a list of SQL
statements.  raw_parser() gains a new argument, enum RawParseMode,
to say what to do.  As proof of concept, add a mode that just parses
a TypeName without any other decoration, and use that to greatly
simplify typeStringToTypeName().

In addition, invent a new SPI entry point SPI_prepare_extended() to
allow SPI users (particularly plpgsql) to get at this new functionality.
In hopes of making this the last variant of SPI_prepare(), set up its
additional arguments as a struct rather than direct arguments, and
promise that future additions to the struct can default to zero.
SPI_prepare_cursor() and SPI_prepare_params() can perhaps go away at
some point.

Discussion: https://postgr.es/m/4165684.1607707277@sss.pgh.pa.us

src/backend/commands/tablecmds.c

@@ -12095,7 +12095,7 @@ ATPostAlterTypeParse(Oid oldId, Oid oldRelId, Oid refRelId, char *cmd,
 	 * parse_analyze() or the rewriter, but instead we need to pass them
 	 * through parse_utilcmd.c to make them ready for execution.
 	 */
-	raw_parsetree_list = raw_parser(cmd);
+	raw_parsetree_list = raw_parser(cmd, RAW_PARSE_DEFAULT);
 	querytree_list = NIL;
 	foreach(list_item, raw_parsetree_list)
 	{