libstdc++: Fix constructor constraints for std::any (PR 90415)

This removes a non-standard extension to std::any which causes errors
for valid code, due to recursive instantiation of a trait that isn't
supposed to be in the constraints.

It also removes some incorrect constraints on the in_place_type<T>
constructors and emplace members, which were preventing creating a
std::any object with another std::any as the contained value.

2020-04-24  Kamlesh Kumar  <kamleshbhalui@gmail.com>
	    Jonathan Wakely  <jwakely@redhat.com>

	PR libstdc++/90415
	PR libstdc++/92156
	* include/std/any (any): Rename template parameters for consistency
	with the standard.
	(any::_Decay): Rename to _Decay_if_not_any.
	(any::any(T&&):: Remove is_constructible from constraints. Remove
	non-standard overload.
	(any::any(in_place_type_t<T>, Args&&...))
	(any::any(in_place_type_t<T>, initializer_list<U>, Args&&...))
	(any::emplace(Args&&...))
	(any::emplace(initializer_list<U>, Args&&...)):
	Use decay_t instead of _Decay.
	* testsuite/20_util/any/cons/90415.cc: New test.
	* testsuite/20_util/any/cons/92156.cc: New Test.
	* testsuite/20_util/any/misc/any_cast_neg.cc: Make dg-error directives
	more robust.
	* testsuite/20_util/any/modifiers/92156.cc: New test.

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