[clang-format] Fix comment levels between '}' and PPDirective

Summary:
This fixes a regression exposed by r307795 in which the level of a comment line
between '}' and a preprocessor directive is incorrectly set as the level of the
line before the '}'. In effect, this:
```
int f(int i) {
  int j = i;
  return i + j;
}
// comment

#ifdef A
#endif
```
was formatted as:
```
int f(int i) {
  int j = i;
  return i + j;
}
  // comment

#ifdef A
#endif
```

Reviewers: djasper

Reviewed By: djasper

Subscribers: cfe-commits, klimek

Differential Revision: https://reviews.llvm.org/D35485

llvm-svn: 308725