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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@236573 91177308-0d34-0410-b5e6-96231b3b80d8