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

[Jonathan Thackray via cfe-commits]

jthackray wrote:

I've opened #132495 to remove quite a lot of superfluous `#include`s, which might improve compile time.

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