[clang] [Clang][NFC] Code cleanup in CGBuiltin.cpp (PR #132060)

[Jonathan Thackray via cfe-commits]

jthackray wrote:

> The timing of this is sort of unfortunate with #132252; the merge conflict there is going to be very painful. @jthackray , thoughts?

@efriedma-quic I think I can update my diff after this is merged; yes, might take a while to resolve the diffs.

https://github.com/llvm/llvm-project/pull/132060