Implements more principled comment parsing.

Changing nextToken() in the UnwrappedLineParser to get the next
non-comment token. This allows us to correctly layout a whole class of
snippets, like:

if /* */(/* */ a /* */) /* */
  f() /* */; /* */
else /* */
  g();

Fixes a bug in the formatter where we would assume there is a previous
non-comment token.
Also adds the indent level of an unwrapped line to the debug output in
the parser.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@173168 91177308-0d34-0410-b5e6-96231b3b80d8