Fixes parsing of hash tokens in the middle of a line.

To parse # correctly, we need to know whether it is the first token in a
line - we can deduct this either from the whitespace or seeing that the
token is the first in the file - we already calculate this information.
This patch moves the identification of the first token into the
getNextToken method and stores it inside the FormatToken, so the
UnwrappedLineParser can stay independent of the SourceManager.