Improve handling of trailing block comments.

We can now (even in non-bin-packing modes) format:
someFunction(1, /* comment 1 */
             2, /* comment 2 */
             3, /* comment 3 */
             aaa);

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@174309 91177308-0d34-0410-b5e6-96231b3b80d8