[Runtime] Replace outdated signature for associated type access functions.

The resolution of generic parameter references, which is used for
checking generic requirements at runtime, was written in terms of an
outdated signature for associated type access functions that did not
account for the MetadataRequest parameter or MetadataResponse result.
Use the existing AssociatedTypeAccessFunction typedef instead.

Fixes SR-7553 / rdar://problem/39769906

Author: DougGregor

stdlib/public/runtime/ProtocolConformance.cpp

@@ -689,12 +689,10 @@ static const Metadata *resolveGenericParamRef(
         protocolDescriptor->getRequirements().data();
 
     // Call the associated type access function.
-    using AssociatedTypeAccessFn =
-      const Metadata *(*)(const Metadata *base, const WitnessTable *);
     unsigned adjustedIndex = index + WitnessTableFirstRequirementOffset;
     current =
-      ((const AssociatedTypeAccessFn *)witnessTable)[adjustedIndex]
-        (current, witnessTable);
+      ((AssociatedTypeAccessFunction * const *)witnessTable)[adjustedIndex]
+        (MetadataState::Abstract, current, witnessTable).Value;
     if (!current) return nullptr;
   }