[HLSL][DirectX] Implement llvm.dx.getDimensions for buffers and lower it to the dxil op

[damyanp]

AC:

• GetDimensions() methods added to the buffer types
• Add builtin, LLVM intrinsic and DXIL op
• Lowering to SPIR-V considered

[hekota]

Split into 2 tasks:
Front end: #112984
Back end: #112982

Closing as duplicate.