Add optional message to unavailablefromasync

Adding the ability to add an optional message to the unavailable from
async attribute. This can be used to indicate other possible API to use,
or help explain why it's unavailable.

lib/Serialization/Serialization.cpp

@@ -2754,6 +2754,16 @@ class Serializer::DeclSerializer : public DeclVisitor<DeclSerializer> {
       TypeSequenceDeclAttrLayout::emitRecord(S.Out, S.ScratchRecord, abbrCode);
       return;
     }
+
+    case DAK_UnavailableFromAsync: {
+      auto abbrCode =
+          S.DeclTypeAbbrCodes[UnavailableFromAsyncDeclAttrLayout::Code];
+      auto *theAttr = cast<UnavailableFromAsyncAttr>(DA);
+      UnavailableFromAsyncDeclAttrLayout::emitRecord(
+          S.Out, S.ScratchRecord, abbrCode, theAttr->isImplicit(),
+          theAttr->Message);
+      return;
+    }
     }
   }