Fix a bug I found while preparing my devmtg talk: When passing NULL to a

function that takes a const Foo&, where Foo is convertible from a large number
of pointer types, we print ALL the overloads, no matter the setting of
-fshow-overloads.

There is potential follow-on work in unifying the "print candidates, but not
too many" logic between OverloadCandidateSet::NoteCandidates and
ImplicitConversionSequence::DiagnoseAmbiguousConversion.

llvm-svn: 167596