c++: fix ICE in joust_maybe_elide_copy [PR106675]

c++: fix ICE in joust_maybe_elide_copy [PR106675]

joust_maybe_elide_copy checks that the last conversion in the ICS for
the first argument is ck_ref_bind, which is reasonable, because we've
checked that we're dealing with a copy/move constructor.  But it can
also happen that we couldn't figure out which conversion function is
better to convert the argument, as in this testcase: joust couldn't
decide if we should go with

  operator foo &()

or

  operator foo const &()

so we get a ck_ambig, which then upsets joust_maybe_elide_copy.  Since
a ck_ambig can validly occur, I think we should just return early, as
in the patch below.

	PR c++/106675

gcc/cp/ChangeLog:

	* call.cc (joust_maybe_elide_copy): Return false for ck_ambig.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/overload-conv-5.C: New test.