embedded: Support class existentials with inherited protocols

For example:
```
protocol Base: AnyObject {}
protocol Derived: Base {}

class C: Derived {}

let e: Derived = C()
```

Author: eeckstein

SwiftCompilerSources/Sources/Optimizer/Utilities/GenericSpecialization.swift

@@ -102,11 +102,16 @@ func specializeWitnessTable(forConformance conformance: Conformance,
         return origEntry
       }
       return .method(requirement: requirement, witness: specializedMethod)
+    case .baseProtocol(let requirement, let witness):
+      let baseConf = context.getSpecializedConformance(of: witness,
+                                                       for: conformance.type,
+                                                       substitutions: conformance.specializedSubstitutions)
+      specializeWitnessTable(forConformance: baseConf, errorLocation: errorLocation, context, notifyNewWitnessTable)
+      return .baseProtocol(requirement: requirement, witness: baseConf)
     default:
       // TODO: handle other witness table entry kinds
       fatalError("unsupported witness table etnry")
     }
-    return origEntry
   }
   let newWT = context.createWitnessTable(entries: newEntries,conformance: conformance,
                                          linkage: .shared, serialized: false)

lib/IRGen/GenProto.cpp

@@ -1563,12 +1563,9 @@ class AccessorConformanceInfo : public ConformanceInfo {
     /// A base protocol is witnessed by a pointer to the conformance
     /// of this type to that protocol.
     void addOutOfLineBaseProtocol(ProtocolDecl *baseProto) {
-#ifndef NDEBUG
       auto &entry = SILEntries.front();
-#endif
       SILEntries = SILEntries.slice(1);
 
-#ifndef NDEBUG
       assert(entry.getKind() == SILWitnessTable::BaseProtocol
              && "sil witness table does not match protocol");
       assert(entry.getBaseProtocolWitness().Requirement == baseProto
@@ -1577,13 +1574,18 @@ class AccessorConformanceInfo : public ConformanceInfo {
       assert((size_t)piIndex.getValue() ==
              Table.size() - WitnessTableFirstRequirementOffset &&
              "offset doesn't match ProtocolInfo layout");
-#endif
 
       // TODO: Use the witness entry instead of falling through here.
 
       // Look for conformance info.
-      auto *astConf = ConformanceInContext.getInheritedConformance(baseProto);
-      assert(astConf->getType()->isEqual(ConcreteType));
+      ProtocolConformance *astConf = nullptr;
+      if (isa<SpecializedProtocolConformance>(SILWT->getConformance())) {
+        astConf = entry.getBaseProtocolWitness().Witness;
+        ASSERT(isa<SpecializedProtocolConformance>(astConf));
+      } else {
+        astConf = ConformanceInContext.getInheritedConformance(baseProto);
+        assert(astConf->getType()->isEqual(ConcreteType));
+      }
       const ConformanceInfo &conf = IGM.getConformanceInfo(baseProto, astConf);
 
       // If we can emit the base witness table as a constant, do so.

test/embedded/existential-class-bound4.swift

@@ -5,8 +5,11 @@
 // REQUIRES: optimized_stdlib
 // REQUIRES: OS=macosx || OS=linux-gnu
 
-protocol ClassBound: AnyObject {
+public protocol Base: AnyObject {
     func foo()
+}
+
+protocol ClassBound: Base {
     func bar()
 }