Note [Always expose compulsory unfoldings]

We must make absolutely sure that unsafeCoerce# is inlined. You might
think that giving it a compulsory unfolding is enough. However,
unsafeCoerce# is put in an interface file just like any other definition.
So, unless we take special precuations
- If we compiled Unsafe.Coerce with -O0, we might not put the unfolding
  into the interface file.
- If we compile a module M, that imports Unsafe.Coerce, with -O0 we might
  not read the unfolding out of the interface file.

So we need to take care, to ensure that Compulsory unfoldings are written
and read.  That makes sense: they are compulsory, after all. There are
three places this is actioned:

* GHC.Iface.Tidy.addExternal.  Export end: expose compulsory
  unfoldings, even with -O0.

* GHC.IfaceToCore.tcIdInfo.  Import end: when reading in from
  interface file, even with -O0 (fignore-interface-pragmas.)  we must
  load a compulsory unfolding