Edit: Do not extend a removal to include trailing whitespace if we're at the end
of the file.

This would run past the end of the buffer. Sadly I don't have a great way to
test it, the only way to trigger the bug is having a removal fix it at the end
of the file, which none of our current warnings can generate.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@217766 91177308-0d34-0410-b5e6-96231b3b80d8