libstdc++: Change names that clash with Win32 or Clang

Clang now defines an __is_unsigned built-in, and Windows defines an
_Out_ macro. Replace uses of those as identifiers.

There might also be a problem with __is_signed, which we use in several
places.

libstdc++-v3/ChangeLog:

	* include/std/chrono (hh_mm_ss): Rename __is_unsigned member to
	_S_is_unsigned.
	* include/std/format (basic_format_context): Rename _Out_
	template parameter to _Out2.
	* testsuite/17_intro/names.cc: Add Windows SAL annotation
	macros.