[clang] [compiler-rt] Define signed int overflow with -fms-compatibility (PR #198538)

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

I think we currently enable `-fno-strict-aliasing` by default for clang-cl?  Not sure if that's tied to -fms-compatibility off the top of my head.

https://github.com/llvm/llvm-project/pull/198538