Format overloaded operators like other functions.

This fixes llvm.org/PR16328 (at least partially).

Before:
SomeLoooooooooooooooooooooooooooooogType operator<<(
    const SomeLooooooooogType &a, const SomeLooooooooogType &b);

After:
SomeLoooooooooooooooooooooooooooooogType
operator<<(const SomeLooooooooogType &a, const SomeLooooooooogType &b);