libstdc++: Fix constexpr memory algo tests for COW std::string

The old COW std::string is not usable in constant expressions, so these
new tests fail with -D_GLIBCXX_USE_CXX11_ABI=0.

The parts of the tests using std::string can be conditionally skipped.

libstdc++-v3/ChangeLog:

	* testsuite/20_util/specialized_algorithms/uninitialized_copy/constexpr.cc:
	Do not test COW std::string in constexpr contexts.
	* testsuite/20_util/specialized_algorithms/uninitialized_default_construct/constexpr.cc:
	Likewise.
	* testsuite/20_util/specialized_algorithms/uninitialized_fill/constexpr.cc:
	Likewise.
	* testsuite/20_util/specialized_algorithms/uninitialized_move/constexpr.cc:
	Likewise.
	* testsuite/20_util/specialized_algorithms/uninitialized_value_construct/constexpr.cc:
	Likewise.

Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>