clang-format: Slightly adapt line break in edge case.

Before:
  SomeMap[std::pair(aaaaaaaaaaaa, bbbbbbbbbbbbbbb)]
      .insert(ccccccccccccccccccccccc);

After:
  SomeMap[std::pair(aaaaaaaaaaaa, bbbbbbbbbbbbbbb)].insert(
      ccccccccccccccccccccccc);

This seems to be about 3:1 more common in Google and Chromium style and I found
only a handful of instances inside the LLVM codebase.