LWG DR 2905 says that is_constructible_v<unique_ptr<P, D>, P, D const &>
should be false when D is not copy constructible. This commit implements
the changes from the DR and simplifies the signatures as per
https://github.com/cplusplus/draft/issues/1530

	* include/bits/unique_ptr.h (__uniq_ptr_impl): Add assertions to
	check deleter type.
	(unique_ptr::unique_ptr(pointer, const deleter_type&)): Add copy
	constructible constraint.
	(unique_ptr::unique_ptr(pointer, deleter_type&&)): Disable for
	deleters of reference type and add move constructible constraint.
	(unique_ptr::unique_ptr(pointer, remove_reference_t<deleter_type>&&)):
	Disable for deleters of non-reference type. Define as deleted.
	(unique_ptr<T[], D>): Likewise.
	* testsuite/20_util/unique_ptr/assign/48635_neg.cc: Replace dg-error
	directives with unstable line numbers with dg-prune-output.
	* testsuite/20_util/unique_ptr/cons/cv_qual_neg.cc: Likewise.
	* testsuite/20_util/unique_ptr/cons/lwg2905.cc: New test.
	* testsuite/20_util/unique_ptr/specialized_algorithms/swap_cxx17.cc:
	Make deleter types invocable.

From-SVN: r264206