PR77987 Fix unique_ptr<T[], D>::reset(U) for T != U

PR77987 Fix unique_ptr<T[], D>::reset(U) for T != U

	PR libstdc++/77987
	* include/bits/unique_ptr.h (unique_ptr<T[], D>::reset<U>(U)): Copy
	value to pointer of the correct type to swap, to support conversions
	allowed by LWG 2118 / N4089.
	* testsuite/20_util/unique_ptr/assign/assign_neg.cc: Move test for
	incompatible deleters from ...
	* testsuite/20_util/unique_ptr/assign/cv_qual.cc: ... here.
	* testsuite/20_util/unique_ptr/modifiers/cv_qual.cc: Move tests for
	incompatible pointers to ...
	* testsuite/20_util/unique_ptr/modifiers/reset_neg.cc: ... here. Move
	destructor definition to base class. Test for invalid derived-to-base
	conversion.

From-SVN: r241235