When overload resolution fails for an overloaded operator, show the

overload candidates (but not the built-in ones). We still rely on the
underlying built-in semantic analysis to produce the initial
diagnostic, then print the candidates following that diagnostic. 

One side advantage of this approach is that we can perform more validation
of C++'s operator overloading with built-in candidates vs. the
semantic analysis for those built-in operators: when there are no
viable candidates, we know to expect an error from the built-in
operator handling code. Otherwise, we are not modeling the built-in
semantics properly within operator overloading. This is checked as:

      assert(Result.isInvalid() && 
             "C++ binary operator overloading is missing
             candidates!");
      if (Result.isInvalid())
        PrintOverloadCandidates(CandidateSet, /*OnlyViable=*/false);

The assert() catches cases where we're wrong in a +Asserts build. The
"if" makes sure that, if this happens in a production clang
(-Asserts), we still build the proper built-in operator and continue
on our merry way. This is effectively what happened before this
change, but we've added the assert() to catch more flies.

llvm-svn: 83175