[HLSL] Implement `Load` method and `operator[]` on structured buffers

[hekota]

Introduce clang builtins (or use existing ones from RWBuffer) to implement Load method and read-only or read/write indexing operator on structured buffers.

Applieds to:

StructuredBuffer
RWStructuredBuffer
RasterizerOrderedStructuredBuffer

The buildings would be lowered to appropriate LLVM DirectX intrinsic during clang codegen.
Depends on #106188.

Docs:
https://learn.microsoft.com/en-us/windows/win32/direct3dhlsl/structuredbuffer-load
https://learn.microsoft.com/en-us/windows/win32/direct3dhlsl/sm5-object-structuredbuffer-operatorindex

AC:

• Load methods and read-only or read/write indexing operators work on structured buffers

[hekota]

Depends on #117018.