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);

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@181585 91177308-0d34-0410-b5e6-96231b3b80d8