clang-format: Fix bug in multiline comment wrapping.

Splitting:
  /**
   * multiline block comment
   *
   */

Before:
  /**
   * multiline block
   *comment
   *
   */

After:
  /**
   * multiline block
   * comment
   *
   */

The reason was that the empty line inside the comment (with just the "*") was
confusing the comment breaking logic.

llvm-svn: 236573