libstdc++: Define std::wstring_convert unconditionally [PR 98725]

libstdc++: Define std::wstring_convert unconditionally [PR 98725]

The wchar_t type is defined unconditionally for C++, so there is no
reason for std::wstring_convert and std::wbuffer_convert to be disabled
when <wchar.h> is not usable. It should be possible to use those class
templates with char16_t and char32_t even if wchar_t conversions don't
work.

libstdc++-v3/ChangeLog:

	PR libstdc++/98725
	* include/bits/locale_conv.h (wstring_convert, wbuffer_convert):
	Define unconditionally. Do not check _GLIBCXX_USE_WCHAR_T.