clang-format: Support member function reference qualifiers.

Before:
  string // break
      operator()() &
  {}

After:
  string // break
  operator()() & {}

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@212041 91177308-0d34-0410-b5e6-96231b3b80d8