glsl/parser: Track built-in types using the glsl_type directly

glsl/parser: Track built-in types using the glsl_type directly

Without the lexer changes, tests/glslparsertest/glsl2/tex_rect-02.frag
fails.  Before this change, the parser would determine that
sampler2DRect is not a valid type because the call to
state->symbols->get_type() in ast_type_specifier::glsl_type() would
return NULL.  Since ast_type_specifier::glsl_type() is now going to
return the glsl_type pointer that it received from the lexer, it doesn't
have an opportunity to generate an error.

   text	   data	    bss	    dec	    hex	filename
8255243	 268856	 294072	8818171	 868dfb	32-bit i965_dri.so before
8255291	 268856	 294072	8818219	 868e2b	32-bit i965_dri.so after
7815195	 345592	 420592	8581379	 82f103	64-bit i965_dri.so before
7815339	 345592	 420592	8581523	 82f193	64-bit i965_dri.so after

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>