clang-format: Several improvements around formatting braced lists.

In particular:
* If the difference between the longest and shortest element, we copped
  out of column format completely. Now, we instead allow to arrange
  these in a single column, essentially enforcing a one-per-line format.
* Allow column layout even if there are braced lists. Especially, if
  there are many short lists, this can be beneficial. The bad case,
  where there is a long nested init list is usually caught as we now
  limit the length difference of the longest and shortest element.

llvm-svn: 236851