PR libstdc++/90220 fix experimental::any_cast for non-object types

PR libstdc++/90220 fix experimental::any_cast for non-object types

This corresponds to the fixes done for std::any_cast, but has to be done
without if-constexpr. The dummy specialization of _Manager_internal<_Op>
is used to avoid instantiating the real _Manager_internal<T>::_S_manage
function just to compare its address.

Backport from mainline
2019-05-23  Jonathan Wakely  <jwakely@redhat.com>

	PR libstdc++/90220
	* include/experimental/any (__any_caster): Constrain to only be
	callable for object types. Use remove_cv_t instead of decay_t.
	If the type decays or isn't copy constructible, compare the manager
	function to a dummy specialization.
	(__any_caster): Add overload constrained for non-object types.
	(any::_Manager_internal<_Op>): Add dummy specialization.
	* testsuite/experimental/any/misc/any_cast.cc: Test function types
	and array types.

From-SVN: r271558