Even better way to handle comments adjacent to preprocessor directives.

Summary:
It turns out that we don't need to store CommentsBeforeNextToken in the
line state, but rather flush them before we start parsing preprocessor
directives. This fixes wrong comment indentation in code blocks in macro calls
(the test is included).

Reviewers: klimek

Reviewed By: klimek

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D617