Fix alignment of trailing block comments.

Summary:
This patch ensures that the lines of the block comments retain relative
column offsets. In order to do this WhitespaceManager::Changes representing
continuation of block comments keep a pointer on the change representing the
whitespace change before the block comment, and a relative column offset to this
change, so that the correct column can be reconstructed at the end of alignment
process.

Fixes http://llvm.org/PR19325

Reviewers: djasper

Reviewed By: djasper

CC: cfe-commits, klimek

Differential Revision: http://reviews.llvm.org/D3408