LWG 3050 Fix cv-qualification of convertibility constraints

LWG 3050 Fix cv-qualification of convertibility constraints

Backport from mainline
2018-06-18  Jonathan Wakely  <jwakely@redhat.com>

	* include/std/chrono (duration, operator*, operator/, operator%): Use
	const-qualified type as source type in is_convertible constraints.
	* testsuite/20_util/duration/arithmetic/dr3050.cc: New.
	* testsuite/20_util/duration/cons/dr3050.cc: New.
	* testsuite/20_util/duration/literals/range.cc: Rename to...
	* testsuite/20_util/duration/literals/range_neg.cc: Here. Adjust
	dg-error lineno.

From-SVN: r263423