[clang-format] Fix regression about short functions after #else

Summary:
This patch fixes a regression introduced in r312904, where the formatter confuses
the `else` in `#else` with an `else` of an `if-else` statement.
For example, formatting this code with google style
```
#ifdef A
int f() {}
#else
int f() {}
#endif
```
resulted in
```
#ifdef A
int f() {}
#else
int f() {
}
#endif
```

Reviewers: sammccall

Reviewed By: sammccall

Subscribers: klimek, cfe-commits

Differential Revision: https://reviews.llvm.org/D37973

llvm-svn: 314683