libstdc++: Make std::type_info::operator== always_inline for C++23 [PR110572]

libstdc++: Make std::type_info::operator== always_inline for C++23 [PR110572]

Commit r12-6266-g3633cc54284450 implemented P1328 for C++23, making
std::type_info::operator== usable in constant expressions. For targets
such as mingw-w64 where that function was not previously inline, making
it constexpr required making it inline for C++23 and later. For
statically linked programs this can result in multiple definition
errors, because there's a non-inline definition in libstdc++.a as well.

For those targets make it always_inline for C++23, so that there is no
symbol generated for the inline definition, and the non-inline
definition in libstdc++.a will be the only definition.

libstdc++-v3/ChangeLog:

	PR libstdc++/110572
	* libsupc++/typeinfo (type_info::operator==): Add always_inline
	attribute for targets where the ABI requries equality to be
	non-inline.
	* testsuite/18_support/type_info/110572.cc: New test.