[HLSL] Introduce a new address space for constant buffer declarations

[hekota]

Introduce a new address space hlsl_constant(2) for constant buffer declarations. Using separate address space will make it easier to distinguish constant globals from other global values. It will be used for all variants of constant buffers :

• cbuffer declarations blocks
• default $Globals constant bufffer
• ConstantBuffer<T>

Clang codegen will generate constant buffer accesses as global variable loads from with hlsl_constant(2) address space. These will be transformed to constant buffer load intrinsics later on in an LLVM pass.