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/tcop/postgres.c

@@ -635,7 +635,7 @@ pg_parse_query(const char *query_string)
 	if (log_parser_stats)
 		ResetUsage();
 
-	raw_parsetree_list = raw_parser(query_string);
+	raw_parsetree_list = raw_parser(query_string, RAW_PARSE_DEFAULT);
 
 	if (log_parser_stats)
 		ShowUsage("PARSER STATISTICS");