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

[Jonathan Thackray via cfe-commits]

jthackray wrote:

So far I've got approvals, positive comments or thumbs-up emojis from: @jhuber6, @erichkeane, @Sirraide, @vmustya, @MacDue, @arichardson.

@s-barannikov and @davemgreen - are you okay if I merge this soon, since `CGBuiltin.cpp` seems to get updated every few days.

I've had to manually merge in changes from #132060 and #132362.

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