[clang-format] Fix regression that breaks comments without a comment prefix

[clang-format] Fix regression that breaks comments without a comment prefix

Summary:
Consider formatting the following code fragment with column limit 20:
```
{
  // line 1
  // line 2\
  // long long long line
}
```
Before this fix the output is:
```
{
  // line 1
  // line 2\
  // long long
  long line
}
```
This patch fixes a regression that breaks the last comment line without
adding the '//' prefix.

Reviewers: djasper

Reviewed By: djasper

Subscribers: cfe-commits, klimek

Differential Revision: https://reviews.llvm.org/D29298

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@293548 91177308-0d34-0410-b5e6-96231b3b80d8