This is an overlooked case in C++17 mandatory copy elision: We want overload
resolution to reflect that initializing an object from a prvalue does not
involve a copy or move constructor even when [over.match.ctor] says that
only constructors are candidates.  Here I implement that by looking through
the copy/move constructor in joust.

	* call.c (joust_maybe_elide_copy): New.
	(joust): Call it.

From-SVN: r269667