[clang-format] Fix reflow in block comment lines with leading whitespace.

Summary:
The reflower was not taking into account the additional  leading whitespace in block comment lines.

source:
```
{
/*
 * long long long long
 *   long
 * long long long long
 */
}
```

format (with column limit 20) before:
```
{
  /*
   * long long long
   * long long long long
   * long long
   */
}
```
format after:
```
{
  /*
   * long long long
   * long long long
   * long long long
   */
}
```

Reviewers: djasper, klimek

Reviewed By: djasper

Subscribers: cfe-commits, sammccall, klimek

Differential Revision: https://reviews.llvm.org/D29326

llvm-svn: 293633