[clang] Introduce `SemaHLSL` (#87912)

This patch introduces `SemaHLSL` class, and moves some HLSL-related
functions there. No functional changes intended.

Removing "HLSL" from function names inside `SemaHLSL` is left for a
subsequent PR by HLSL contributors, if they deem that desirable.

This is a part of the effort to split `Sema` into smaller manageable
parts, and follows the example of OpenACC. See #82217, #84184, #87634
for additional context.