clang-format: Fix bug in column-layout formatting.

Specific arrangements of comments after trailing commas could confuse
the column width calculation, e.g. in:

vector<int> x = { a, b,
                  /* some */ /* comment */ };

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@189211 91177308-0d34-0410-b5e6-96231b3b80d8