[clang] [NFC][clang] Split clang/lib/CodeGen/CGBuiltin.cpp into target-specific files (PR #132252)

[Farzon Lotfi via cfe-commits]

farzonl wrote:

In the future It would have been good to get feedback from the HLSL team before moving `EmitHLSLBuiltinExpr` to `clang/lib/CodeGen/TargetBuiltins/AMDGPU.cpp`. HLSL is a language and does not fall into the TargetBuiltin categorization. It supports all gpus.

https://github.com/llvm/llvm-project/pull/132252