glsl: Drop the round-trip through ast_type_specifier for many builtin types.

glsl: Drop the round-trip through ast_type_specifier for many builtin types.

We have lexer recognition of a bunch of our types based on the
handling.  This code was mapping those recognized tokens to an enum
and then to a string of their name.  Just drop the enums and provide
the string directly in the parser.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>