[clang] [llvm] [CodeGen][Clang] Add -fstack-protector-layout-only (PR #231651)

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

In the shadow-stack case, do we need to force a spill of the return address if there are objects that need a stack protector?

More generally, I think I'd like to tie the flag more strongly to the specific target feature that's providing the protection.  If some target feature is providing a form of "stack protection", we need to make sure that feature is actually spilling the register in question in the right place.  The implementation might be target-independent in some sense, but it's relying on invariants that the backend might not provide in the general case.

https://github.com/llvm/llvm-project/pull/231651