[clang] [clang] Ensure correct copying of records with authenticated fields (PR #136783)

[Oliver Hunt via cfe-commits]

ojhunt wrote:

> Do we need to do anything special for constant expression handling?

I do not believe so as pointer auth does not occur during consteval, but I'll see if we have any additional existing tests that I haven't included here. If not I'll add some new ones:

- [ ] Type checking (e.g. pointer auth is a no op in constexpr, but `void*__ptrauth(a)*` is not the same as `void*__ptrauth(b)*`)
- [ ] constexpr struct copied into another struct should have correct signing



https://github.com/llvm/llvm-project/pull/136783