This is not required by the standard, but seems useful.

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>

libstdc++-v3/ChangeLog:

	* include/std/utility (exchange): Add noexcept-specifier.
	* testsuite/20_util/exchange/noexcept.cc: New test.

Tested powerpc64le-linux. Committed to trunk.