Sema: Don't validate class layout when validating an extension

We only need to validate all the members of a class when
we type check an extension, which means we will generate
code for it.

Merely validating an extension, as happens during name
lookup, does not need to validate all members of a class.

Author: slavapestov

lib/Sema/TypeCheckDecl.cpp

@@ -6906,6 +6906,9 @@ class DeclChecker : public DeclVisitor<DeclChecker> {
       if (auto extendedTy = ED->getExtendedType()) {
         if (auto nominal = extendedTy->getAnyNominal()) {
           TC.validateDecl(nominal);
+          if (auto *classDecl = dyn_cast<ClassDecl>(nominal))
+            TC.requestClassLayout(classDecl);
+
           // Check the raw values of an enum, since we might synthesize
           // RawRepresentable while checking conformances on this extension.
           if (auto enumDecl = dyn_cast<EnumDecl>(nominal)) {
@@ -8180,8 +8183,6 @@ void TypeChecker::validateExtension(ExtensionDecl *ext) {
   // Validate the nominal type declaration being extended.
   auto nominal = extendedType->getAnyNominal();
   validateDecl(nominal);
-  if (auto *classDecl = dyn_cast<ClassDecl>(nominal))
-    requestClassLayout(classDecl);
 
   if (nominal->getGenericParamsOfContext()) {
     auto genericParams = ext->getGenericParams();