[PATCH] [lld] add calls to nativereader/nativewriter

[Shankar Kalpathi Easwaran]

  That would work for passes that want to replace a Mutable File with another MutableFile.

  The problem with merging the ReaderWriterNative and ReaderWriterYAML is they parse files to lld:::File's and we need a down conversion to a MutableFile.

http://llvm-reviews.chandlerc.com/D1955