GLSL builtin variables

[tly000]

• Define builtin headers for all shader types
• look at libclcxx/include/opencl_workitem how to define builtin variables
• use llvm triple (i.e. spir-unknown-glsl-vertex) to default-include correct builtins
  • maybe define preprocessor macros (i.e. __vertex_shader__, __fragment_shader__ etc.)

[tly000]

TODOS:

• gl_PerVertex blocks ( gl_in and gl_out )
• constants (i.e. gl_WorkGroupSize)