libstdc++: Disable std::array assertions for C++11 constexpr

libstdc++: Disable std::array assertions for C++11 constexpr

The recent changes to add assertions to std::array broke the functions
that need to be constexpr in C++11, because of the restrictive rules for
constexpr functions in C++11.

This simply disables the assertions for C++11 mode, so the functions can
be constexpr again.

libstdc++-v3/ChangeLog:

	* include/std/array (array::operator[](size_t) const, array::front() const)
	(array::back() const) [__cplusplus == 201103]: Disable
	assertions.
	* testsuite/23_containers/array/element_access/constexpr_element_access.cc:
	Check for correct values.
	* testsuite/23_containers/array/tuple_interface/get_neg.cc:
	Adjust dg-error line numbers.
	* testsuite/23_containers/array/debug/constexpr_c++11.cc: New test.