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

[Farzon Lotfi via cfe-commits]

farzonl wrote:

Would it be ok if we prefixed all of these files in `TargetBuiltins` with CG? https://github.com/llvm/llvm-project/issues/133199

Building using AppleClang seems to be upset that a cpp file with this name already exists. I'm assuming its talking about all the files in `clang/lib/Basic/Targets/`. For example there is now a `clang/lib/Basic/Targets/AMDGPU.cpp` and `clang/lib/CodeGen/TargetBuiltins/AMDGPU.cpp`.

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