Replace SimpleFileWrapper with a function.

SimpleFileWrapper was a class to wrap an existing (possibly non-mutable)
file as a mutable file. We used instances of the class in RoundTrip*
passes, because the passes convert mutable files to non-mutable files,
and we needed to convert them back to mutable.

That feature can be implemented without defining a new class. Generally
speaking, if we can implement a feature without defining a class and
using only public interface of exsiting classes, that's preferred way
to do that. And this is the case.

llvm-svn: 227549