glsl: Don't allow invalid identifier names in struct/interface fields.

glsl: Don't allow invalid identifier names in struct/interface fields.

Note: we need to make an exception for the gl_PerVertex interface
block, since built-in variables are allowed to be redeclared inside
it.  Future patches will make redeclaration of gl_PerVertex work
properly.

Fixes piglit tests:
- spec/glsl-1.50/compiler/interface-block-array-elem-uses-gl-prefix.vert
- spec/glsl-1.50/compiler/named-interface-block-elem-uses-gl-prefix.vert
- spec/glsl-1.50/compiler/unnamed-interface-block-elem-uses-gl-prefix.vert

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