[NFC][libcxxabi] Run clang-format on libcxxabi/src/cxa_guard_impl.h

I'm about to submit a change which involves re-writing most of
cxa_guard_impl.h. Running clang-format on the whole file first seems like a
good idea.

Reviewed By: ldionne, #libc_abi

Differential Revision: https://reviews.llvm.org/D108231