[cppyy] Deal with Clang attributes like `_Nonnull`

[guitargeek]

Apple's libc++ headers (especially in newer macOS/Xcode SDKs) annotate some std::basic_string<wchar_t> constructors with Clang nullability qualifiers (e.g. const wchar_t* _Nonnull).

The cppyy backend currently preserves these qualifiers in the type spelling, so the existing wchar_t* converter does not match wchar_t*_Nonnull. This breaks Python string to std::wstring conversions and causes STL string tests to fail on macOS.

In #21348, we added an explicit converter mapping for wchar_t*_Nonnull to reuse the existing WCStringConverter. A more general solution would be to strip/normalize Clang attributes during type matching.

[dpiparo]

Did the PR address this issue?

[aaronj0]

Did the PR address this issue?

Looks like it did, the cppyy stl tests passed the nightlies on mac-beta

[guitargeek]

Yes, but it's just a workaround for the specific type where we saw the problem. If libc++ uses these attributes in more places in the future, it will become a problem again. So it would be better to deal with these attributes more systematically, and this PR is to remind us of that.

[dpiparo]

I see, thanks