[clang] [Clang][X86] Introduce Clang ABI Gate for MSVC alignment (PR #210305)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> IIRC we haven't been gating MSVC-compat ABI changes behind -fclang-abi-compat in the past. Do you have object files where this actually matters?

Didn't we do that for vector deleting destructors because we had people who needed both ABIs? CC @Fznamznon 

That said, we typically only add an ABI compat flag if there's either user demand for it or if there's an up-front signal that it's likely to be needed (regardless of MSVC compat or otherwise).

https://github.com/llvm/llvm-project/pull/210305