[clang-format] Enable comment reflowing in multiline comments containing pragmas

Summary:
This patch enables comment reflowing of lines not matching the comment pragma regex
in multiline comments containing comment pragma lines. Previously, these comments
were dumped without being reindented to the result.

Reviewers: djasper, mprobst

Reviewed By: mprobst

Subscribers: klimek, mprobst, cfe-commits

Differential Revision: https://reviews.llvm.org/D30697

llvm-svn: 297261