Fix a latent bug in propagateSoleConformingType: check reabstraction.

This code was force casting the address of the opened existential to
the lowered SIL type of the known conformance. This is superficially
incorrect because values are stored in existentials with maximal
reabstraction.

Bail on any concrete types that require reabstraction. I don't think
we will hit this currently because there is an earlier check that
prevents optimizing a conformance on a generic types. In theory
someone could add that functionality later for internal generic types
with a single instantiation.

More importantly, we don't want anyone copying this logic and assuming
it's generally correct.

Author: atrick

lib/SILOptimizer/SILCombiner/SILCombinerApplyVisitors.cpp

@@ -693,6 +693,14 @@ SILCombiner::buildConcreteOpenedExistentialInfoFromSoleConformingType(
     return COAI;
   }
   if (auto *OEA = dyn_cast<OpenExistentialAddrInst>(OAI.OpenedArchetypeValue)) {
+    // Bail if ConcreteSILType is not the same SILType as the type stored in the
+    // existential after maximal reabstraction.
+    auto archetype = OpenedArchetypeType::getAny(SoleCEI.ExistentialType);
+    Lowering::AbstractionPattern abstractionPattern(archetype);
+    auto abstractTy = M.Types.getLoweredType(abstractionPattern, ConcreteType);
+    if (abstractTy != concreteSILType)
+       return None;
+
     SoleCEI.ConcreteValue =
       Builder.createUncheckedAddrCast(
         OEA->getLoc(), OEA, concreteSILType.getAddressType());