Fix treatment of hi-boot files and dfuns

Trac #16038 exposed the fact that TcRnDriver.checkHiBootIface
was creating a binding, in the module being compiled, for
   $fxBlah = $fBlah

 but $fxBlah was a /GlobalId/. But all bindings should be for
 /LocalIds/ else dependency analysis goes down the tubes.

* I added a CoreLint check that an occurrence of a GlobalId
  is not bound by an binding of a LocalId.  (There is already
  a binding-site check that no binding binds a GlobalId.)

* I refactored (and actually signficantly simplified) the
  tricky code for dfuns in checkHiBootIface to ensure that
  we get LocalIds for those boot-dfuns.

Alas, I then got "duplicate instance" messages when compiling
HsExpr. It turns out that this is a long-standing, but extremely
delicate, bug: even before this patch, if you compile HsExpr
with -ddump-tc-trace, you get "duplicate instance". Without
-ddump-tc-trace, it's OK.  What a mess!

The reason for the duplicate-instance is now explained in
Note [Loading your own hi-boot file] in LoadIface.  I fixed
it by a Gross Hack in LoadIface.loadInterface. This is at
least no worse than before.

But there should be a better way. I have opened #16081 for this.

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