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

[Jonathan Thackray via cfe-commits]

jthackray wrote:

> I'd say you can merge it once CI is green.

Thanks @jhuber6.

CI is green. clang-format isn't, but running clang-format breaks some of the AArch64 tests.

I'll wait another few hours, just in case there are more comments, and merge later today if not.

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