libstdc++: -D_GLIBCXX_DEBUG fixes in the constrained algos tests

libstdc++: -D_GLIBCXX_DEBUG fixes in the constrained algos tests

This fixes the failures in the constrained algos tests when they are run in
debug mode.

libstdc++-v3/ChangeLog:

	* testsuite/25_algorithms/copy/constrained.cc: Don't assume that the
	base() of a vector<>::iterator is a pointer.
	* testsuite/25_algorithms/copy_backward/constrained.cc: Likewise.
	* testsuite/25_algorithms/move/constrained.cc: Likewise.
	* testsuite/25_algorithms/move_backward/constrained.cc: Likewise.
	* testsuite/25_algorithms/inplace_merge/constrained.cc: Use foo.data()
	instead of &foo[0].
	* testsuite/25_algorithms/partial_sort/constrained.cc: Likewise.
	* testsuite/25_algorithms/partial_sort_copy/constrained.cc: Likewise.
	* testsuite/25_algorithms/shuffle/constrained.cc: Likewise.
	* testsuite/25_algorithms/sort/constrained.cc: Likewise.
	* testsuite/25_algorithms/stable_sort/constrained.cc: Likewise.