clang-format: Improve <<-formatting.

This fixes a regression caused by r186115.

Before:
  Diag(aaaaaaaaaaaaaaaaaaaaaaaaaaaaa,
       bbbbbbbbb) << aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
                  << aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa;

After:
  Diag(aaaaaaaaaaaaaaaaaaaaaaaaaaaaa, bbbbbbbbb)
      << aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
      << aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa;