Fix bug when formatting overloaded operators.

Before, the actual operator of an overloaded operator declaration was
handled as a binary operator an thus, clang-format could not find valid
formattings for many examples, e.g.:

template <typename AAAAAAA, typename BBBBBBB>
AAAAAAA operator/(const AAAAAAA &a, BBBBBBB &b);

llvm-svn: 181585