re PR libstdc++/49151 ([C++0x][constexpr] chrono::duration has incorrect non-member operator semantics)

2011-05-24  Paolo Carlini  <paolo.carlini@oracle.com>

	PR libstdc++/49151
	* include/std/chrono (operator+, operator-, operator*, operator/,
	operator&): Implement LWG 2020 [WP]; specify constexpr.
	* testsuite/20_util/duration/arithmetic/dr2020.cc: New.

From-SVN: r174150