Allow locale{nullptr} to compile

[StephanTLavavej]

Found by std/localization/locales/locale/locale.cons/char_pointer.pass.cpp in the upcoming libcxx update:

D:\GitHub\STL\llvm-project\libcxx\test\std\localization\locales\locale\locale.cons\char_pointer.pass.cpp(92): error C2440: '<function-style-cast>': cannot convert from 'nullptr' to 'std::locale'
D:\GitHub\STL\llvm-project\libcxx\test\std\localization\locales\locale\locale.cons\char_pointer.pass.cpp(92): note: 'std::locale::locale': ambiguous call to overloaded function
D:\GitHub\STL\out\x64\out\inc\xlocale(304): note: could be 'std::locale::locale(const char *,std::locale::category)'
D:\GitHub\STL\out\x64\out\inc\xlocale(394): note: or       'std::locale::locale(std::locale::_Locimp *)'
D:\GitHub\STL\llvm-project\libcxx\test\std\localization\locales\locale\locale.cons\char_pointer.pass.cpp(92): note: while trying to match the argument list '(nullptr)'

WG21-N4964 [member.functions]/2:

For a non-virtual member function described in the C++ standard library, an implementation may declare a different set of member function signatures, provided that any call to the member function that would select an overload from the set of declarations described in this document behaves as if that overload were selected.

The set of constructors depicted in [locale.general] means that locale{nullptr} should compile, even though [locale.cons]/3 says that when it selects explicit locale(const char* std_name);:

Throws: runtime_error if the argument is not valid, or is null.

libc++'s test suite expects exactly this.

C++ is intentionally designed to perform access control after overload resolution (citation left as an exercise to the reader), so the private constructor locale(_Locimp*) disrupts this. We can add an internal tag to restore conformance. Because this is private, the tag can also be private.

Verified that there are no differences in release or debug exports (perhaps surprisingly, locale isn't separately compiled, although its unindicted co-conspirators are).

[StephanTLavavej]

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.