Revert "[clang] Add frontend flag to enable support for broken external resugarers (#103219)"

This partially reverts commit 661dda9.

The flag itself is left with no effect, to be removed in a later patch,
in order to help with the transition.

Author: mizvekov

clang/lib/Sema/SemaTemplate.cpp

@@ -3531,16 +3531,10 @@ QualType Sema::CheckTemplateIdType(TemplateName Name,
     if (Pattern->isInvalidDecl())
       return QualType();
 
-    // Only substitute for the innermost template argument list.  NOTE: Some
-    // external resugarers rely on leaving a Subst* node here.  Make the
-    // substitution non-final in that case.  Note that these external resugarers
-    // will still miss some information in this representation, because we don't
-    // provide enough context in the Subst* nodes in order to tell different
-    // template type alias specializations apart.
+    // Only substitute for the innermost template argument list.
     MultiLevelTemplateArgumentList TemplateArgLists;
-    TemplateArgLists.addOuterTemplateArguments(
-        Template, CTAI.SugaredConverted,
-        /*Final=*/!getLangOpts().RetainSubstTemplateTypeParmTypeAstNodes);
+    TemplateArgLists.addOuterTemplateArguments(Template, CTAI.SugaredConverted,
+                                               /*Final=*/true);
     TemplateArgLists.addOuterRetainedLevels(
         AliasTemplate->getTemplateParameters()->getDepth());