Fix FMT_NO_UNIQUE_ADDRESS warning with clang-cl.

[lukester1975]

Hi

clang-cl is generating a warning with the no_unique_address change from c86fe0b: warning : unknown attribute 'no_unique_address' ignored [-Wunknown-attributes]

Thread on clang-cl and no_unique_address: https://reviews.llvm.org/D110485

Here's an additional check check for clang-cl (plus use __has_cpp_attribute(msvc::no_unique_address) to cope in case clang-cl adds support; cl returns true since 19.31).

Thanks

[vitaut]

__has_cpp_attribute -> FMT_HAS_CPP_ATTRIBUTE

[vitaut]

Actually, I don't think we need an attribute check since the branch below only handles specific compiler (versions).

[lukester1975]

OK, FMT_HAS_CPP_ATTRIBUTE done, missed that.

The reason I put the msvc::no_unique_address attr check is to future proof if support gets added to clang-cl, rather than always not setting.

I suppose if clang-cl waits to enable the standard no_unique_address until after cl does and never bothers with the msvc attr it'll be moot, but this way at least it won't need revising either way... Or at least that was the intention!

[vitaut]

I don't think we should be adding support for some not-standard attribute that may or may not be supported in the future. Let's remove the attribute check.

[lukester1975]

OK sure, done.

Seems a shame to miss out on the size optimization if it does get added, though! Not that I've looked at where it's used in fmtlib 😀

[vitaut]

Thanks