clang-format: Break before/between array subscript expressions.

clang-format used to treat array subscript expressions much like
function call (just replacing () with []). However, this is not really
appropriate especially for expressions with multiple subscripts.

Although it might seem counter-intuitive, the most consistent solution
seems to be to always (if necessary) break before a square bracket,
never after it. Also, multiple subscripts of the same expression should
be aligned if they are on subsequent lines.

Before:
  aaaaaaaaaaaaaaaaaaaaaaaaa[aaaaaaaaaaaaaaaaaaaaaaaaa][
      bbbbbbbbbbbbbbbbbbbbbbbbb] = c;
  aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa[
      aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa][
      bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb] = ccccccccccc;

After:
  aaaaaaaaaaaaaaaaaaaaaaaaa[aaaaaaaaaaaaaaaaaaaaaaaaa]
                           [bbbbbbbbbbbbbbbbbbbbbbbbb] = c;
  aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
      [aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa]
      [bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb] = ccccccccccc;

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@186153 91177308-0d34-0410-b5e6-96231b3b80d8