[clang-format] Fix regression merging comments across newlines.

Summary:
This fixes a regression that causes example:
```
enum A {
  a, // line a

  // line b
  b
};
```
to be formatted as follows:
```
enum A {
  a, // line a
     // line b
  b
};
```

Reviewers: djasper, klimek

Reviewed By: klimek

Subscribers: cfe-commits, sammccall, djasper, klimek

Differential Revision: https://reviews.llvm.org/D29322

llvm-svn: 293624