libstdc++: Do not define __cpp_lib_constexpr_exceptions [PR121114]

libstdc++: Do not define __cpp_lib_constexpr_exceptions [PR121114]

Do not advertise library support for constexpr exceptions, as our
solution to throwing by __throw_* functions from <bits/functexcept.h>,
caues constant evaluation to fail, as these functions are not constexpr.

	PR libstdc++/121114

libstdc++-v3/ChangeLog:

	* include/bits/version.def (constexpr_exceptions): Add no_stdname
	and changed value.
	* include/bits/version.h: Regenerated.
	* testsuite/18_support/exception/version.cc: Test that macro is
	not exported.

Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
Signed-off-by: Tomasz Kaminski <tkaminsk@redhat.com>