c++: Avoid calling const copy ctor on implicit move. [PR91212]

c++: Avoid calling const copy ctor on implicit move. [PR91212]

Our implementation of C++11 implicit move was wrong for return; we didn't
actually hit the check for the type of the first parameter of the selected
constructor, because we didn't see LOOKUP_PREFER_RVALUE set properly.

Fixing that to look at the right flags fixed the issue for this testcase,
but broke implicit move for a by-value converting constructor (PR58051).  I
think this was not allowed in C++17, but it is allowed under the implicit
move changes from C++20, and those changes were voted to apply as a DR to
earlier standards as well, so I don't want to break it now.

So after fixing the flags check I changed the test to allow value
parameters.

gcc/cp/ChangeLog:

	PR c++/91212
	* call.c (build_over_call): Don't call a const ref
	overload for implicit move.

gcc/testsuite/ChangeLog:

	PR c++/91212
	* g++.dg/cpp0x/move-return3.C: New test.