Make the format scrambler understand line comments.

This allows for writing tests including line comments easier and more readable.
We will need more of those tests in the future and also line comments are
useful to force line breaks in tests.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@170446 91177308-0d34-0410-b5e6-96231b3b80d8