libtdc++: Define std::source_location for C++20

libtdc++: Define std::source_location for C++20

This doesn't define a new _GLIBCXX_HAVE_BUILTIN_SOURCE_LOCATION macro.
because using __has_builtin(__builtin_source_location) is sufficient.
Currently only GCC supports it, but if/when Clang and Intel add it the
__has_builtin check should for them too.

Co-authored-by: Jonathan Wakely <jwakely@redhat.com>
libstdc++-v3/ChangeLog:

	* doc/doxygen/user.cfg.in (INPUT): Add <source_location>.
	* include/Makefile.am: Add <source_location>.
	* include/Makefile.in: Regenerate.
	* include/std/version (__cpp_lib_source_location): Define.
	* include/std/source_location: New file.
	* testsuite/18_support/source_location/1.cc: New test.
	* testsuite/18_support/source_location/consteval.cc: New test.
	* testsuite/18_support/source_location/srcloc.h: New test.
	* testsuite/18_support/source_location/version.cc: New test.