Clang defines the __cpp_impl_destroying_delete macro unconditionally, so
that the feature is supported whenever the library type is defined. This
is incompatible with the current definition in libstdc++ because we use
constexpr and inline variables, which will give an error for older -std
modes.

This patch defines the destroying_delete_t type and destroying_delete
variable independently of the __cpp_impl_destroying_delete macro, but
only for C++2a (because the names aren't reserved for previous
standards). The __cpp_lib_destroying_delete macro is only defined when
both the library type and compiler macro are defined (i.e. when the type
can actually be used as intended).

	PR libstdc++/89345
	* include/std/version [__cpp_impl_destroying_delete]
	(__cpp_lib_destroying_delete): Only define for C++2a and later.
	* libsupc++/new [__cpp_impl_destroying_delete]
	(__cpp_lib_destroying_delete): Likewise.
	(destroying_delete_t, destroying_delete): Likewise, but define even
	when __cpp_impl_destroying_delete is not defined.
	* testsuite/18_support/destroying_delete.cc: New test.

From-SVN: r268856