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

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

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

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