clang-format: Fix bug in function ref qualifier identification.

.. and simplify it.

Before:
  void A::f()&& {}
  void f() && {}

After:
  void A::f() && {}
  void f() && {}

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@272124 91177308-0d34-0410-b5e6-96231b3b80d8