[clang] [X86] Return illegal vectors in memory (PR #121944)

[Phoebe Wang via cfe-commits]

phoebewang wrote:

This is a long existing problem and LLVM community doesn't make consensus about it, see https://reviews.llvm.org/D53919

My point is the case of returning illegal vectors is not well defined by psABI, and we have already warned this case in frontend. Considering the backward compatibility risk, I'd like to keep it as it.

https://github.com/llvm/llvm-project/pull/121944