Fix bug in C++ overload resolution

Fix bug in C++ overload resolution

PR c++/28901 points out a bug in C++ overload resolution.  When
comparing two overloads, one might be better than the other for
certain parameters -- but, if that one also has some invalid
conversion, then it should never be considered the better choice.
Instead, a valid-but-not-apparently-quite-as-good overload should be
preferred.

This patch fixes this problem by changing how overload comparisons are
done.  I don't believe it should affect any currently valid overload
resolution; nor should it affect resolutions where all the choices are
equally invalid.