libstdc++: Implement std::ios_base::noreplace for C++23 [PR59769]

libstdc++: Implement std::ios_base::noreplace for C++23 [PR59769]

This implements my P2467R0 proposal to support opening an fstream in
exclusive mode. The new constant is also supported pre-C++23 as
std::ios_base::__noreplace.

This proposal hasn't been approved for C++23 yet, but I am confident it
will be, as this is restoring a feture found in pre-ISO C++ iostreams
implementations (and still present in the MSVC library as _Noreplace).
If the proposal fails for C++23 we can remove the ios::noreplace
name and just keep ios::__noreplace as an extension.

libstdc++-v3/ChangeLog:

	PR libstdc++/59769
	* config/io/basic_file_stdio.cc (fopen_mode): Add support for
	exclusive mode.
	* include/bits/ios_base.h (_S_noreplace): Define new enumerator.
	(ios_base::__noreplace): Define.
	(ios_base::noreplace): Define for C++23.
	* include/std/version (__cpp_lib_ios_noreplace): Define.
	* testsuite/27_io/basic_ofstream/open/char/noreplace.cc: New test.
	* testsuite/27_io/basic_ofstream/open/wchar_t/noreplace.cc: New test.