Fix up the typechecking of interface files during --make
This patch fixes Trac #909. The problem was that when compiling the base package, the handling of wired-in things wasn't right; in TcIface.tcWiredInTyCon it repeatedly loaded GHC.Base.hi into the PIT, even though that was the very module it was compiling. The main fix is by introducing TcIface.ifCheckWiredInThing. But I did some minor refactoring as well.