Runtime checking for associated types conforming to invertible protocols

Emit metadata for runtime checks of conformances of associated types to
invertible protocols, e.g., `T.Assoc: Copyable`. This allows us to
correctly handle, e.g., dynamic casting involving conditional
conformances that have such constraints.

The model we use here is to emit an invertible-protocol constraint
that leaves only the specific bit clear in the invertible protocol
set.

lib/SILOptimizer/Utils/CastOptimizer.cpp

@@ -17,6 +17,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "swift/SILOptimizer/Utils/CastOptimizer.h"
+#include "swift/AST/ExistentialLayout.h"
 #include "swift/AST/GenericSignature.h"
 #include "swift/AST/Module.h"
 #include "swift/AST/SubstitutionMap.h"
@@ -1472,6 +1473,10 @@ static bool optimizeStaticallyKnownProtocolConformance(
     if (Conformance.isInvalid())
       return false;
 
+    auto layout = TargetType->getExistentialLayout();
+    if (layout.getProtocols().size() != 1)
+      return false;
+
     SILBuilderWithScope B(Inst);
     SmallVector<ProtocolConformanceRef, 1> NewConformances;
     NewConformances.push_back(Conformance);