[clang-format] Break before a sequence of line comments aligned with the next line.

Summary:
Make the comment alignment respect sections of line comments originally alinged
with the next token. Until now the decision how to break a continuous sequence
of line comments into sections was taken without reference to the next token.

source:
```
class A {
public: // comment about public
  // comment about a
  int a;
}
```

format before:
```
class A {
public: // comment about public
        // comment about a
  int a;
}
```

format after:
```
class A {
public: // comment about public
  // comment about a
  int a;
}
```

Reviewers: djasper, klimek

Reviewed By: klimek

Subscribers: cfe-commits, klimek

Differential Revision: https://reviews.llvm.org/D29626

llvm-svn: 294435