[clang-format] Keep level of comment before an empty line

Summary:
This patch fixes bug https://bugs.llvm.org/show_bug.cgi?id=3313: a comment line
was aligned with the next #ifdef even in the presence of an empty line between
them.

Reviewers: djasper, klimek

Reviewed By: djasper

Subscribers: klimek, cfe-commits

Differential Revision: https://reviews.llvm.org/D35296

llvm-svn: 307795