[SE-0497] Implement @export attribute syntax

Implement the @export(implementation) and @export(interface) attributes
to replace @_alwaysEmitIntoClient and @_neverEmitIntoClient. Provide a
warning + Fix-It to start staging out the very-new
@_neverEmitIntoClient. We'll hold off on pushing folks toward
@_alwaysEmitIntoClient for a little longer.

lib/Serialization/Serialization.cpp

@@ -3070,6 +3070,14 @@ class Serializer::DeclSerializer : public DeclVisitor<DeclSerializer> {
       return;
     }
 
+    case DeclAttrKind::Export: {
+      auto *theAttr = cast<ExportAttr>(DA);
+      auto abbrCode = S.DeclTypeAbbrCodes[ExportDeclAttrLayout::Code];
+      ExportDeclAttrLayout::emitRecord(S.Out, S.ScratchRecord, abbrCode,
+                                       (unsigned)theAttr->exportKind);
+      return;
+    }
+        
     case DeclAttrKind::NonSendable: {
       auto *theAttr = cast<NonSendableAttr>(DA);
       auto abbrCode = S.DeclTypeAbbrCodes[NonSendableDeclAttrLayout::Code];