[parser] Only clear tempzone if we actually parse in it

The previous clear mechanism would also unnecessarily allocate a preparser.

This fixes regressions introduced by
https://chromium-review.googlesource.com/c/v8/v8/+/1238614

Change-Id: Ia3eef371c80857f2cd6cdd5852c849fa7f8716fc
Reviewed-on: https://chromium-review.googlesource.com/1238918
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56171}

src/ast/scopes.h

@@ -102,7 +102,6 @@ class V8_EXPORT_PRIVATE Scope : public NON_EXPORTED_BASE(ZoneObject) {
   void SetScopeName(const AstRawString* scope_name) {
     scope_name_ = scope_name;
   }
-  void set_needs_migration() { needs_migration_ = true; }
 #endif
 
   // TODO(verwaest): Is this needed on Scope?
@@ -684,7 +683,12 @@ class V8_EXPORT_PRIVATE DeclarationScope : public Scope {
   }
   bool is_being_lazily_parsed() const { return is_being_lazily_parsed_; }
 #endif
-  void set_zone(Zone* zone) { zone_ = zone; }
+  void set_zone(Zone* zone) {
+#ifdef DEBUG
+    needs_migration_ = true;
+#endif
+    zone_ = zone;
+  }
 
   bool ShouldEagerCompile() const;
   void set_should_eager_compile();