Improve formatting of code with comments.

Before:
aaaaaaa(aaaaaa( // comment
                aaaaaaa));
<big mess>

After:
aaaaaaa(aaaaaa( // comment
    aaaaaaaa));
function(/* parameter 1 */ aaaaaaa,
         /* parameter 2 */ aaaaaaa,
         /* parameter 3 */ aaaaaaa,
         /* parameter 4 */ aaaaaaa);

(the latter example was only wrong in the one-arg-per-line mode, e.g. in
Google style).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@173821 91177308-0d34-0410-b5e6-96231b3b80d8