[IRGen] Implement resilient access pattern for associated conformances.

When referencing an associated conformance in a witness table for a
resilient protocol, use the associated conformance descriptor to compute
the index into the witness table at run-time.

Another part of rdar://problem/44167982.

lib/IRGen/GenDecl.cpp

@@ -3387,6 +3387,16 @@ llvm::GlobalValue *IRGenModule::defineAssociatedTypeDescriptor(
   return defineAlias(entity, definition);
 }
 
+llvm::Constant *IRGenModule::getAddrOfAssociatedConformanceDescriptor(
+                                          AssociatedConformance conformance) {
+  auto entity = LinkEntity::forAssociatedConformanceDescriptor(
+                  conformance.getSourceProtocol(),
+                  conformance.getAssociation(),
+                  conformance.getAssociatedRequirement());
+  return getAddrOfLLVMVariable(entity, getPointerAlignment(), ConstantInit(),
+                               ProtocolRequirementStructTy, DebugTypeInfo());
+}
+
 llvm::GlobalValue *IRGenModule::defineAssociatedConformanceDescriptor(
                                                 ProtocolDecl *proto,
                                                 CanType subject,