[IRGen] Restore the old code path for emitting existential type metadata

for plain protocol and protocol composition types.

These types should always be wrapped in ExistentialType, but there isn't
sufficient validation of this throughout the compiler yet. Change
the fatal error when the metadata request sees these plain types to an
assert and restore the old type metadata emission path for protocol and
protocol composition types to avoid crashing in those cases.

Author: hborla

lib/IRGen/MetadataRequest.cpp

@@ -1883,7 +1883,16 @@ namespace {
 
     MetadataResponse visitProtocolType(CanProtocolType type,
                                        DynamicMetadataRequest request) {
-      llvm_unreachable("constraint type should be wrapped in existential type");
+      assert(false && "constraint type should be wrapped in existential type");
+
+      CanExistentialType existential(
+          ExistentialType::get(type)->castTo<ExistentialType>());
+
+      if (auto metatype = tryGetLocal(existential, request))
+        return metatype;
+
+      auto metadata = emitExistentialTypeMetadata(existential);
+      return setLocal(type, MetadataResponse::forComplete(metadata));
     }
 
     MetadataResponse
@@ -1892,7 +1901,16 @@ namespace {
       if (type->isAny() || type->isAnyObject())
         return emitSingletonExistentialTypeMetadata(type);
 
-      llvm_unreachable("constraint type should be wrapped in existential type");
+      assert(false && "constraint type should be wrapped in existential type");
+
+      CanExistentialType existential(
+          ExistentialType::get(type)->castTo<ExistentialType>());
+
+      if (auto metatype = tryGetLocal(existential, request))
+        return metatype;
+
+      auto metadata = emitExistentialTypeMetadata(existential);
+      return setLocal(type, MetadataResponse::forComplete(metadata));
     }
 
     MetadataResponse