Also refactor some constraints slightly to be more readable.

	PR libstdc++/90165
	* include/std/variant (variant::__not_self): New helper for the
	is_same_v<remove_cvref_t<T>, variant>==false constraints.
	(variant::__to_type_impl): Remove.
	(variant::__to_type): Add default argument to check pack size, instead
	of using __to_type_impl.
	(variant::__accepted_type): Add default argument using __not_self.
	(variant::__is_in_place_tag, variant::__not_in_place_tag): New helpers
	for variant(T&&) constructor constraint.
	(variant::variant(T&&)): Use __not_in_place_tag in constraints.
	Extract __accepted_type into a named template parameter for reuse in
	other constraints and in the exception specification.
	(variant::variant(in_place_type_t<T>, Args&&...))
	(variant::variant(in_place_type_t<T>, initializer_list<U>, Args&&...))
	(variant::variant(in_place_index_t<T>, Args&&...))
	(variant::variant(in_place_index_t<T>, initializer_list<U>, Args&&...))
	(variant::operator=T&&)): Remove redundant && from trait arguments.
	* testsuite/20_util/variant/compile.cc: Check variant(T&&) constructor
	isn't used for in_place_type or in_place_index arguments.

Tested powerpc64le-linux, committed to trunk.

I'll consider a simpler version of this for gcc-8-branch, that just
applies the __not_in_place_tag constraint to the variant(T&&) ctor.