[Codable] When explicit existential types are enabled, synthesize Codable

initializer requirements with explicit existential Encodable and Decodable
parameters.

Author: hborla

Diff for: lib/Sema/DerivedConformanceCodable.cpp

@@ -1212,6 +1212,9 @@ static FuncDecl *deriveEncodable_encode(DerivedConformance &derived) {
   // Create from the inside out:
 
   auto encoderType = C.getEncoderType();
+  if (C.LangOpts.EnableExplicitExistentialTypes)
+    encoderType = ExistentialType::get(encoderType);
+
   auto returnType = TupleType::getEmpty(C);
 
   // Params: (Encoder)
@@ -1803,6 +1806,9 @@ static ValueDecl *deriveDecodable_init(DerivedConformance &derived) {
 
   // Params: (Decoder)
   auto decoderType = C.getDecoderType();
+  if (C.LangOpts.EnableExplicitExistentialTypes)
+    decoderType = ExistentialType::get(decoderType);
+
   auto *decoderParamDecl = new (C) ParamDecl(
       SourceLoc(), SourceLoc(), C.Id_from,
       SourceLoc(), C.Id_decoder, conformanceDC);