libstdc++: Make std::construct_at support arrays (LWG 3436)

The issue was approved at the recent St. Louis meeting, requiring
support for bounded arrays, but only without arguments to initialize the
array elements.

libstdc++-v3/ChangeLog:

	* include/bits/stl_construct.h (construct_at): Support array
	types (LWG 3436).
	* testsuite/20_util/specialized_algorithms/construct_at/array.cc:
	New test.
	* testsuite/20_util/specialized_algorithms/construct_at/array_neg.cc:
	New test.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/initlist-opt1.C: Adjust for different diagnostics
	from std::construct_at by adding -fconcepts-diagnostics-depth=2.