Fix aligning of comments.

Previously we started sequences to align for single line comments when
the previous line had a trailing comment, but the sequence was broken
for other reasons.

Now we re-format:
// a
 // b
f(); // c
to:
// a
// b
f(); // c

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@182608 91177308-0d34-0410-b5e6-96231b3b80d8