PR libstdc++/85951 for make_signed/make_unsigned for character types

PR libstdc++/85951 for make_signed/make_unsigned for character types

Because the wide character types are neither signed integer types nor
unsigned integer types they need to be transformed to an integral type
of the correct size and the lowest rank (which is not necessarily the
underlying type). Reuse the helpers for enumeration types to select the
correct integer.

The refactoring of __make_unsigned_selector and __make_signed_selector
slightly reduces the number of template instantiations and so reduces
memory usage.

	PR libstdc++/85951
	* include/std/type_traits [_GLIBCXX_USE_C99_STDINT_TR1]: Do not define
	uint_least16_t and uint_least32_t.
	(__make_unsigned<wchar_t>): Define unconditionally.
	(__make_unsigned_selector<_Tp, true, false>): Remove intermediate
	typedefs.
	(__make_unsigned_selector_base): New type to provide helper templates.
	(__make_unsigned_selector<_Tp, false, true>): Reimplement using
	__make_unsigned_selector_base helpers.
	(__make_unsigned<char16_t>, __make_unsigned<char32_t>): Define.
	(__make_signed_selector<_Tp, true, false>): Remove intermediate
	typedefs.
	(__make_signed<wchar_t>, __make_signed<char16_t>)
	(__make_signed<char32_t>)): Define unconditionally.
	* testsuite/20_util/make_signed/requirements/typedefs-3.cc: Check
	wchar_t, char16_t and char32_t are transformed correctly.
	* testsuite/20_util/make_signed/requirements/typedefs_neg.cc: Adjust
	dg-error lineno.
	* testsuite/20_util/make_unsigned/requirements/typedefs-3.cc: Check
	wchar_t, char16_t and char32_t are transformed correctly.
	* testsuite/20_util/make_unsigned/requirements/typedefs_neg.cc: Adjust
	dg-error lineno.

From-SVN: r261023