[SE-0258] Promote projection variables ($foo) to the original property access

When the outermost property wrapper associated with a property has a
`wrapperValue`, create the projection property (with the `$` prefix)
at the same access level as the original property. This puts the
wrapped-value interface and the projection interface at the same level.

The newly-introduced @_projectionValueProperty attribute is implicitly
created to establish the link between the original property and the
projection value within module interfaces, where both properties will
be explicitly written out.

lib/Serialization/Serialization.cpp

@@ -2510,6 +2510,17 @@ class Serializer::DeclSerializer : public DeclVisitor<DeclSerializer> {
         S.addTypeRef(theAttr->getTypeLoc().getType()));
       return;
     }
+
+    case DAK_ProjectionValueProperty: {
+      auto abbrCode =
+          S.DeclTypeAbbrCodes[ProjectionValuePropertyDeclAttrLayout::Code];
+      auto theAttr = cast<ProjectionValuePropertyAttr>(DA);
+      ProjectionValuePropertyDeclAttrLayout::emitRecord(
+        S.Out, S.ScratchRecord, abbrCode, theAttr->isImplicit(),
+        S.addDeclBaseNameRef(theAttr->ProjectionPropertyName));
+      break;
+    }
+
     }
   }