[clang-format] comment reflow: add last line's penalty when ending broken

Summary:
This fixes a bug in clang-format where the last line's penalty is not
taken into account when its ending is broken. Usually the last line's penalty
is handled by addNextStateToQueue, but in cases where the trailing `*/` is put
on a newline, the contents of the last line have to be considered for penalizing.

Reviewers: mprobst

Reviewed By: mprobst

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D50378

llvm-svn: 339123