Refactor comment merging.

- We scan for whitespace between comments anyways, remember any newlines seen
  along the way.
- Use this newline number to decide whether two comments are adjacent.
- Since the newline check is now free remove the caching and unused code.
- Remove unnecessary boolean state from the comment list.
- No behavioral change.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@191614 91177308-0d34-0410-b5e6-96231b3b80d8