[HLSL][NFC] Refactor resource buffers AST tests (llvm#117659)

This is a test-only change. This PR merges 5 tests for structured
buffers ASTs into one file with multiple run lines and check prefixes
because these tests have a lot of common output. It also renames
RWBuffer-AST.hlsl to TypedBuffers-AST.hlsl and makes it ready to include
testing of other typed buffers as well.

Fixes llvm#115412

Author: hekota