[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? #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`.
> 
> We currently have four, so I'm unsure why one more makes a difference. I guess because they go in the same library component or something?
> 
> ```
> clang/lib/Basic/Targets/AMDGPU.cpp
> clang/lib/CodeGen/TargetBuiltins/AMDGPU.cpp
> clang/lib/CodeGen/Targets/AMDGPU.cpp
> clang/lib/Driver/ToolChains/AMDGPU.cpp
> ```

I don't know what libtool is doing here to give you a confident answer.

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