clang-format: Fix bug in wrapping behavior of operators.

Before (even violating the column limit):
  auto Diag =
      diag()
      << aaaaaaaaaaaaaaaa(aaaaaaaaaaaa, aaaaaaaaaaaaa, aaaaaaaaaaaaaaaaaaaaaaaaaa);

After:
  auto Diag = diag() << aaaaaaaaaaaaaaaa(aaaaaaaaaaaa, aaaaaaaaaaaaa,
                                         aaaaaaaaaaaaaaaaaaaaaaaaaa);

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@297931 91177308-0d34-0410-b5e6-96231b3b80d8