array (array::at, [...]): Mark constexpr.

2011-07-20  Benjamin Kosnik  <bkoz@redhat.com>
	    Daniel Krugler  <daniel.kruegler@googlemail.com>

	* include/std/array (array::at, array::operator[]): Mark constexpr.
	* testsuite/23_containers/array/requirements/
	constexpr_element_access.cc: Add.


Co-Authored-By: Daniel Krugler <daniel.kruegler@googlemail.com>

From-SVN: r176550