glsl: check for forced_language_version in is_version()
glsl: check for forced_language_version in is_version()

This is a follow-on fix from the earlier "glsl: allow ForceGLSLVersion
to override #version directives" change.  Since we're not changing
the language_version field, we have to check forced_language_version
here.

Reviewed-by: Ian Romanick <2b237cafb16dc45038e85df6c85e74e6d899eba9@intel.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler