[HLSL] Add `StructuredBuffer` definition to HLSLExternalSemaSource

[bogner]

Create the StructuredBuffer template in HLSLExternalSemaSource, which each contain a handle that will be lowered to a dx.RawBuffer target extension type with a struct as their type argument.

[damyanp]

Blocked on llvm/wg-hlsl#68.

[hekota]

Fixed by #106316

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Justin Bogner (bogner)

Create the `StructuredBuffer` template in HLSLExternalSemaSource, which each contain a handle that will be lowered to a `dx.RawBuffer` target extension type with a struct as their type argument.