[Rewrite][NFC] Add FIXMEs and tests for RemoveLineIfEmpty bug

I'd like to add these comments to warn others of problems I
encountered when trying to use `RemoveLineIfEmpty`.  I originally
tried to fix the problem, but I realized I could implement the
functionality more easily and efficiently in my calling code where I
can make the simplifying assumption that there are no prior edits to
the line from which text is being removed.  While I've lost the
motivation to write a fix, which doesn't look easy, I figure a warning
to others is better than silence.

I've added a unit test to demonstrate the problem.  I don't know how
to mark it as an expected failure, so I just marked it disabled.

Reviewed By: jkorous

Differential Revision: https://reviews.llvm.org/D61466

llvm-svn: 369049

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