clang-format: Improve line wrapping around << operators.

Generally, clang-format tries to keep label-value pairs on a single
line for stream operators. However, we should not do that if there is
just a single such pair, as that doesn't help much.

Before:
  llvm::errs() << "aaaaaaaaaaaa: " << aaaaaaa(aaaaaaaaa,
                                              aaaaaaaaa);

After:
  llvm::errs() << "aaaaaaaaaaaa: "
               << aaaaaaa(aaaaaaaaa, aaaaaaaaa);

Also remove old test case that was testing actual behavior any more.

llvm-svn: 237535