libstdc++: Define type traits for wchar_t even when libc support missing

This meets the requirement that std::is_integral_v<wchar_t> is true,
even when full library support for wchar_t via specializations of
char_traits etc. is not provided. This is done by checking
__WCHAR_TYPE__ to see if the compiler knows about the type, rather than
checking the library's own _GLIBCXX_USE_WCHAR_T autoconf macro.

This assumes that the C++ compiler correctly defines wchar_t as a
distinct type, not a typedef for one of the other integeral types. This
is always true for G++ and should be true for any supported non-GNU
compilers.

Similarly, the std::make_unsigned and std::make_signed traits and the
internal helpers std::__is_integer and std::__is_char are also changed
to depend on the same macro.

libstdc++-v3/ChangeLog:

	* include/std/type_traits (is_integral<wchar_t>)
	(make_unsigned<wchar_t>, make_signed<wchar_t>): Define based
	on #ifdef __WCHAR_TYPE__ instead of _GLIBCXX_USE_WCHAR_T.
	* include/bits/cpp_type_traits.h (__is_integer<wchar_t>)
	(__is_char<wchar_t>): Likewise.

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