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

[Shilei Tian via cfe-commits]

https://github.com/shiltian commented:

I'm super happy to see this change. The AMDGPU part looks good to me! Thanks!

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