Get rid of DiscardedInfo, and fix a Ghci bug at the same time.

The new story is this:

- We always read the whole interface file, as it exists on disk,
  not dropping pragmas or anything.

- We compare that from-the-disk copy with the new version before
  writing the new interface file.

- We drop the pragmas
  a) Before loading the interface payload into the declaration pools
  b) In the no-need-to-recompile case, before typechecking the
	interface decls.  Omitting this was the previous bug.