Fixed long-standing issue with incorrect length calculation of multi-line comments.

Summary:
A trailing block comment having multiple lines would cause extremely
high penalties if the summary length of its lines is more than the column limit.
Fixed by always considering only the last line of a multi-line block comment.
Removed a long-standing FIXME from relevant tests and added a motivating test
modelled after problem cases from real code.

Reviewers: klimek

Reviewed By: klimek

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D1010