clang-format: Fix incorrect trailing return arrow detection.

Before:
  auto doSomething(Aaaaaa* aaaaaa) -> decltype(aaaaaa -> f()) {}

After:
  auto doSomething(Aaaaaa* aaaaaa) -> decltype(aaaaaa->f()) {}

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@220373 91177308-0d34-0410-b5e6-96231b3b80d8