PR libstdc++/91308 fix constraints on unique_ptr assignment

PR libstdc++/91308 fix constraints on unique_ptr assignment

	* testsuite/20_util/unique_ptr/assign/48635_neg.cc: Replace dg-error
	with dg-prune-output for enable_if failure.
	* testsuite/20_util/unique_ptr/cons/cv_qual_neg.cc: Add
	dg-prune-output for enable_if failure.

Backport from mainline
2019-07-31  Jonathan Wakely  <jwakely@redhat.com>

	PR libstdc++/91308
	* include/bits/unique_ptr.h (unique_ptr::__safe_conversion_up): Remove
	constraints on deleter that should only apply to the constructor.
	(unique_ptr<T[], D>::__safe_conversion_up): Likewise.
	(unique_ptr<T[], D>::unique_ptr(unique_ptr<U, D>&&)): Restore
	constraints on deleter here.
	* testsuite/20_util/unique_ptr/assign/91308.cc: New test.

From-SVN: r275311