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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> > Another example is a potential [..]
> 
> This does sound hypothetical.

I don't think it's hypothetical, we've got internal reports at Intel about this breaking oneMKL (a math library). It took a while for our automated testing to catch it and I think we can work around it internally, but I would expect other libraries to be hit similarly. (FWIW, the debugging experience was about what you'd expect for an ABI change.)

I don't insist on a particular direction, but I'm okay with adding the ABI flag in principle.

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