[clang-format] Fix breaking of comment sections in unwrapped lines containing newlines.

Summary:
The breaking of line comment sections was misaligning the case where the first comment line is on an unwrapped line containing newlines. In this case, the breaking column must be based on the source column of the last token that is preceded by a newline, not on the first token of the unwrapped line.

source:
```
enum A {
  a, // line 1
  // line 2
};
```
format before:
```
enum A {
  a, // line 1
     // line 2
};
```
format after:
```
enum A {
  a, // line 1
  // line 2
};
```

Reviewers: djasper

Reviewed By: djasper

Subscribers: cfe-commits, klimek

Differential Revision: https://reviews.llvm.org/D29444

llvm-svn: 293891