Autogen: Restore mocs_compilation in OBJECT libraries

Since commit f65f20938c (Autogen: Avoid processing CSharp targets,
2020-11-12, v3.20.0-rc1~301^2) we collect all sources for a target
earlier than previously.  Clear the sources cache so that it will be
re-computed later after AUTOGEN processing.

Fixes: #22085

Source/cmQtAutoGenGlobalInitializer.cxx

@@ -97,6 +97,11 @@ cmQtAutoGenGlobalInitializer::cmQtAutoGenGlobalInitializer(
       }
       std::set<std::string> const& languages =
         target->GetAllConfigCompileLanguages();
+      // cmGeneratorTarget::GetAllConfigCompileLanguages caches the target's
+      // sources. Clear it so that OBJECT library targets that are AUTOGEN
+      // initialized after this target get their added mocs_compilation.cpp
+      // source acknowledged by this target.
+      target->ClearSourcesCache();
       if (languages.count("CSharp")) {
         // Don't process target if it's a CSharp target
         continue;