[NFC][HLSL] Move emitter out of AMDGPU.cpp (#133251)

- Move all HLSL code out of AMDGPU.cpp to CGHLSLBuiltins.cpp
- Fixes accidental reorganization of HLSL code into AMDGPU caused by
(#132252,
7f920e2e5f70b)

Author: farzonl

.github/new-prs-labeler.yml

@@ -637,6 +637,7 @@ hlsl:
   - clang/lib/Sema/HLSLExternalSemaSource.cpp
   - clang/lib/Sema/SemaHLSL.cpp
   - clang/lib/CodeGen/CGHLSLRuntime.*
+  - clang/lib/CodeGen/CGHLSLBuiltins.cpp
   - llvm/include/llvm/Frontend/HLSL/**
   - llvm/lib/Frontend/HLSL/**