[clang] [CIR] Fix problem with phantom function arguments (PR #140322)

[Andy Kaylor via cfe-commits]

andykaylor wrote:

> LGTM. Is there an easy to add a test for this? I understand sometimes it's hard, but just double checking.

The only thing I can think of would be a unit test that calculates folding set IDs for a bunch of similar, but slightly different, function signatures to verify that we don't incorrectly match them. That's probably worth doing, but I wouldn't want to hold this fix for it.

https://github.com/llvm/llvm-project/pull/140322