c++: Tuple of self-dependent classes [PR96926]

c++: Tuple of self-dependent classes [PR96926]

When compiling this testcase, trying to resolve the initialization for the
tuple member ends up recursively considering the same set of tuple
constructor overloads, and since two of them separately depend on
is_constructible, the one we try second fails to instantiate
is_constructible because we're still in the middle of instantiating it the
first time.

Fixed by implementing an optimization that someone suggested we were already
doing: if we see a non-template candidate that is a perfect match for all
arguments, we can skip considering template candidates at all.  It would be
enough to do this only when LOOKUP_DEFAULTED, but it shouldn't hurt in other
cases.

gcc/cp/ChangeLog:

	PR c++/96926
	* call.c (perfect_conversion_p): New.
	(perfect_candidate_p): New.
	(add_candidates): Ignore templates after a perfect non-template.

gcc/testsuite/ChangeLog:

	PR c++/96926
	* g++.dg/cpp0x/overload4.C: New test.