[clang][AST] Handle overload callee type in CallExpr::getCallReturnType.

The function did not handle every case. In some cases this
caused assertion failure.
After the fix the function returns DependentTy if the exact
return type can not be determined.

It seems that clang itself does not call the function in the
affected cases but some checker or other code may call it.

Reviewed By: hokein

Differential Revision: https://reviews.llvm.org/D95244