[AST] Use CustomAttr's DeclContext for ResolveMacroRequest

Remove the DeclContext parameter from ResolveMacroRequest, we can now
retrieve the DeclContext either from the CustomAttr or macro expansion
expr/decl directly.

lib/Serialization/Serialization.cpp

@@ -3304,7 +3304,7 @@ class Serializer::DeclSerializer : public DeclVisitor<DeclSerializer> {
       auto theAttr = cast<CustomAttr>(DA);
 
       // Macro attributes are not serialized.
-      if (D->getResolvedMacro(const_cast<CustomAttr *>(theAttr)))
+      if (theAttr->getResolvedMacro())
         return;
 
       auto attrType =