Produce good looking diagnostics on ambiguous built-in operators.

Now we produce things like:
bug1.cpp:21:11: error: use of overloaded operator '->*' is ambiguous
        int i = c->*pmf;        // expected-error {{use of overloaded operator '->*' is ambiguous}} \
                ~^  ~~~
bug1.cpp:21:11: note: built-in candidate operator ->* ('struct A volatile *', 'int const struct A::*')
bug1.cpp:21:11: note: built-in candidate operator ->* ('struct A volatile *', 'int restrict struct A::*')
...

Still need to look at an issue (indicated as FIXME in the test case).

llvm-svn: 83650