clang-format: More eagerly wrap trailing return types.

Before:
  template <typename T>
  auto aaaaaaaaaaaaaaaaaaaaaa(T t) -> decltype(eaaaaaaaaaaaaaaa<T>(t.a)
                                                   .aaaaaaaa());

After:
  template <typename T>
  auto aaaaaaaaaaaaaaaaaaaaaa(T t)
      -> decltype(eaaaaaaaaaaaaaaa<T>(t.a).aaaaaaaa());

Also add a test case for a difficult template parsing case I stumbled accross.
Needs fixing.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@239149 91177308-0d34-0410-b5e6-96231b3b80d8