[clang] [llvm] Implement `preserve_none` for 32-bit x86 (PR #150106)

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

For "nest", we can forbid combining it with the preserves_none calling convention, probably, as long as we can detect it and error out.  There's no reason anyone would combine the two.

For the base pointer, you also need to worry about the callee-save register list: I don't think we have code to properly save/restore the base pointer if it gets clobbered by a call.

https://github.com/llvm/llvm-project/pull/150106