libstdc++: Fix wstring conversions in filesystem::path [PR95048]

In commit r9-7381-g91756c4a I changed filesystem::path to use
std::codecvt<CharT, char, mbstate_t> for conversions from all wide
strings to UTF-8, instead of using std::codecvt_utf8<CharT>. This was
done because for 16-bit wchar_t, std::codecvt_utf8<wchar_t> only
supports UCS-2 and not UTF-16. The rationale for the change was sound,
but the actual fix was not. It's OK to use std::codecvt for char16_t or
char32_t, because the specializations for those types always use UTF-8 ,
but std::codecvt<wchar_t, char, mbstate_t> uses the current locale's
encodings, and the narrow encoding is probably ASCII and can't support
non-ASCII characters.

The correct fix is to use std::codecvt only for char16_t and char32_t.
For 32-bit wchar_t we could have continued using std::codecvt_utf8
because that uses UTF-32 which is fine, switching to std::codecvt broke
non-Windows targets with 32-bit wchar_t. For 16-bit wchar_t we did need
to change, but should have changed to std::codecvt_utf8_utf16<wchar_t>
instead, as that always uses UTF-16 not UCS-2. I actually noted that in
the commit message for r9-7381-g91756c4a but didn't use that
option. Oops.

This replaces the unconditional std::codecvt<CharT, char, mbstate_t>
with a type defined via template specialization, so it can vary
depending on the wide character type. The code is also simplified to
remove some of the mess of #ifdef and if-constexpr conditions.

libstdc++-v3/ChangeLog:

	PR libstdc++/95048
	* include/bits/fs_path.h (path::_Codecvt): New class template
	that selects the kind of code conversion done.
	(path::_Codecvt<wchar_t>): Select based on sizeof(wchar_t).
	(_GLIBCXX_CONV_FROM_UTF8): New macro to allow the same code to
	be used for Windows and POSIX.
	(path::_S_convert(const EcharT*, const EcharT*)): Simplify by
	using _Codecvt and _GLIBCXX_CONV_FROM_UTF8 abstractions.
	(path::_S_str_convert(basic_string_view<value_type>, const A&)):
	Simplify nested conditions.
	* include/experimental/bits/fs_path.h (path::_Cvt): Define
	nested typedef controlling type of code conversion done.
	(path::_Cvt::_S_wconvert): Use new typedef.
	(path::string(const A&)): Likewise.
	* testsuite/27_io/filesystem/path/construct/95048.cc: New test.
	* testsuite/experimental/filesystem/path/construct/95048.cc: New
	test.