clang-format: Add special case to reduce indentaiton in streams.

This is similar to how we treat assignments and seems to be generally
desirable.

Before:
  llvm::errs() << aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa(
                      aaaaaaaaaaaaaaaaaaaaaaaaaaaa,
                      aaaaaaaaaaaaaaaaaaaaaaaaaaaa);

After:
  llvm::errs() << aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa(
      aaaaaaaaaaaaaaaaaaaaaaaaaaaa, aaaaaaaaaaaaaaaaaaaaaaaaaaaa);