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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