Sema: Clean up named opaque result type generic signature construction

We were building the signature twice, and adding the 'where' clause twice
each time.

The GSB magically uniqued them, whereas the Requirement Machine is not
so forgiving.

lib/Serialization/Deserialization.cpp

@@ -3567,6 +3567,8 @@ public:
     auto genericSig = MF.getGenericSignature(genericSigID);
     if (genericSig)
       opaqueDecl->setGenericSignature(genericSig);
+    else
+      opaqueDecl->setGenericSignature(GenericSignature());
     if (underlyingTypeSubsID) {
       auto subMapOrError = MF.getSubstitutionMapChecked(underlyingTypeSubsID);
       if (!subMapOrError)