clang-format: Improve column layout.

Specifically, calculate the deviation between the shortest and longest
element (which is used to prevent excessive whitespace) per column, not
overall. This automatically handles the corner cases of a single column
and a single row so that the actualy implementation becomes simpler.

Before:
  vector<int> x = {1,
                   aaaaaaaaaaaaaaaaaaaaaa,
                   2,
                   bbbbbbbbbbbbbbbbbbbbbb,
                   3,
                   cccccccccccccccccccccc};

After:
  vector<int> x = {1, aaaaaaaaaaaaaaaaaaaaaa,
                   2, bbbbbbbbbbbbbbbbbbbbbb,
                   3, cccccccccccccccccccccc};

llvm-svn: 236992