clang-format: Fix segfault in overloaded operator parsing.

Before, constructs like:
  using A::operator+;

caused a segfault. This fixes llvm.org/PR17050.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@189749 91177308-0d34-0410-b5e6-96231b3b80d8