glsl: Don't allow invalid identifiers as interface block instance names.

glsl: Don't allow invalid identifiers as interface block instance names.

Note: we need to make an exception for the gl_PerVertex interface
block, since in geometry shaders it is allowed to be redeclared with
the instance name gl_in.  Future patches will make redeclaration of
gl_PerVertex work properly.

Fixes piglit test
spec/glsl-1.50/compiler/interface-block-instance-name-uses-gl-prefix.vert.

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