[clang] Add flag to suppress overflow errors in C++ constant expressions. (PR #102390)

[Hubert Tong via cfe-commits]

hubert-reinterpretcast wrote:

I don't see a compelling reason for the extension and I find its behaviour somewhat arbitrary: why is it okay to allow the overflow in the definition of an enumerator but not okay to allow for `alignas`?

https://github.com/llvm/llvm-project/pull/102390