[SPIRV] support builtin types and ExtInsts selection

The patch adds the support of OpenCL and SPIR-V built-in types. It also
implements ExtInst selection and adds spv_unreachable and spv_alloca
intrinsics which improve the generation of the corresponding SPIR-V code.
Five LIT tests are included to demonstrate the improvement.

Differential Revision: https://reviews.llvm.org/D132648



Co-authored-by: Aleksandr Bezzubikov <zuban32s@gmail.com>
Co-authored-by: Michal Paszkowski <michal.paszkowski@outlook.com>
Co-authored-by: Andrey Tretyakov <andrey1.tretyakov@intel.com>
Co-authored-by: Konrad Trifunovic <konrad.trifunovic@intel.com>