Handle overloaded operators in ?: precedence warning

This is a follow-up to r132565, and should address the rest of PR9969:

Warn about cases such as

int foo(A a, bool b) {
 return a + b ? 1 : 2; // user probably meant a + (b ? 1 : 2);
}

also when + is an overloaded operator call.

llvm-svn: 132784