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.

libstdc++-v3/libsupc++/typeinfo

@@ -188,6 +188,9 @@ namespace std
 #endif
 
 #if __GXX_TYPEINFO_EQUALITY_INLINE || __cplusplus > 202002L
+# if ! __GXX_TYPEINFO_EQUALITY_INLINE
+  [[__gnu__::__always_inline__]]
+# endif
   _GLIBCXX23_CONSTEXPR inline bool
   type_info::operator==(const type_info& __arg) const _GLIBCXX_NOEXCEPT
   {

libstdc++-v3/testsuite/18_support/type_info/110572.cc

+// { dg-options "-static-libstdc++" }
+// { dg-require-static-libstdcxx }
+// { dg-require-cpp-feature-test __cpp_rtti }
+// { dg-do link }
+
+#include <typeinfo>
+
+int main()
+{
+  return typeid(0) == typeid(0u);
+}