[clang] [Clang] Ensure "=default"ed function can be deleted when used as an extension in C++03 (PR #90725)

[Nikolas Klauser via cfe-commits]

philnik777 wrote:

Note that this is also a problem with an implicit assignment operator: https://godbolt.org/z/jrh5novMo. I'm not 100% sure it's not a bug, but it definitely looks to me like one. It's definitely a mismatch between C++03 and C++11, which I wouldn't expect given that `= delete` a C++11 extension.

https://github.com/llvm/llvm-project/pull/90725