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

Summary:
This fixes a regression exposed by r307795 and rL308725 in which the level of a
comment line between '} else {' and a preprocessor directive is incorrectly set
as the level of the '} else {' line. For example, this :
```
int f(int i) {
  if (i) {
    ++i;
  } else {
    // comment
#ifdef A
    --i;
#endif
  }
}
```
was formatted as:
```
int f(int i) {
  if (i) {
    ++i;
  } else {
  // comment
#ifdef A
    --i;
#endif
  }
}
```

Reviewers: djasper, klimek

Reviewed By: klimek

Subscribers: klimek, cfe-commits

Differential Revision: https://reviews.llvm.org/D35794

llvm-svn: 308882