Fix aligning of comments that are at the start of the line.

Now correctly leaves:
f(); // comment
// comment
g(); // comment
... alone if the middle comment was aligned with g() before formatting.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@182605 91177308-0d34-0410-b5e6-96231b3b80d8