[SPIRV-V] Add SPIR-V logical triple to llc

This commits adds the minimal required bits to build a logical SPIR-V
compute shader using LLC.
- Skip OpenCL-only capabilities & extensions for Logical SPIR-V.
- Generate required metadata for entrypoints from HLSL frontend.
- Fix execution mode to GLCompute in logical.

The main issue is the lack of "vulkan" bit in the triple.
This might need to be added as a vendor?
Because as-is, SPIRV32/64 assumes OpenCL, and then, SPIRV assumes
Vulkan. This is ok-ish today, but not correct.

Differential Revision: https://reviews.llvm.org/D156424