[Runtime] Add "reserved" protocol conformance kind and ignore it.

The protocol conformance record has two bits to describe how the
witness table will be produced. There are currently three states
(direct reference to witness table, witness table accessor, and
conditional witness table accessor). Add a reserved case for the
fourth state so the Swift 5 runtime will (silently) ignore
conformances using that fourth state, when/if some future Swift
uses it.

Author: DougGregor

include/swift/ABI/MetadataValues.h

@@ -220,6 +220,8 @@ enum class ProtocolConformanceReferenceKind : unsigned {
   /// table whose conformance is conditional on additional requirements that
   /// must first be evaluated and then provided to the accessor function.
   ConditionalWitnessTableAccessor,
+  /// Reserved for future use.
+  Reserved,
 };
 
 // Type metadata record discriminant

include/swift/Runtime/Metadata.h

@@ -2665,6 +2665,9 @@ struct TargetProtocolConformanceRecord {
     case ProtocolConformanceReferenceKind::WitnessTableAccessor:
     case ProtocolConformanceReferenceKind::ConditionalWitnessTableAccessor:
       assert(false && "not witness table");
+
+    case ProtocolConformanceReferenceKind::Reserved:
+      break;
     }
     return WitnessTable;
   }
@@ -2673,6 +2676,7 @@ struct TargetProtocolConformanceRecord {
     switch (Flags.getConformanceKind()) {
     case ProtocolConformanceReferenceKind::WitnessTableAccessor:
     case ProtocolConformanceReferenceKind::ConditionalWitnessTableAccessor:
+    case ProtocolConformanceReferenceKind::Reserved:
       break;
 
     case ProtocolConformanceReferenceKind::WitnessTable:

stdlib/public/runtime/ProtocolConformance.cpp

@@ -90,6 +90,9 @@ template<> void ProtocolConformanceRecord::dump() const {
       printf("conditional witness table accessor %s\n",
              symbolName((const void *)(uintptr_t)getWitnessTableAccessor()));
       break;
+    case ProtocolConformanceReferenceKind::Reserved:
+      printf("reserved witness table kind\n");
+      break;
   }
 }
 #endif
@@ -165,6 +168,9 @@ const {
             typeName.c_str(), demangledProtocolName.c_str());
     return nullptr;
   }
+
+  case ProtocolConformanceReferenceKind::Reserved:
+    return nullptr;
   }
 
   swift_runtime_unreachable(
@@ -591,6 +597,11 @@ swift::swift_conformsToProtocol(const Metadata * const type,
           C.cacheSuccess(type, P, record.getWitnessTable(type));
           break;
 
+        case ProtocolConformanceReferenceKind::Reserved:
+          // Always fail, because we cannot interpret a future conformance
+          // kind.
+          C.cacheFailure(metadata, P);
+          break;
         }
       }
     }