Re-apply "Sema: Targeted fix for LLDB REPL issue"

Now with a change to the AST printer to never print @_fixed_layout.
Once some more groundwork is in place, we will be able to only print
this attribute when its needed, but this is good enough for now.

Author: slavapestov

include/swift/AST/PrintOptions.h

@@ -123,7 +123,8 @@ struct PrintOptions {
   bool PrintUserInaccessibleAttrs = true;
 
   /// List of attribute kinds that should not be printed.
-  std::vector<DeclAttrKind> ExcludeAttrList = { DAK_Transparent, DAK_Effects };
+  std::vector<DeclAttrKind> ExcludeAttrList =
+      { DAK_Transparent, DAK_Effects, DAK_FixedLayout };
 
   /// List of attribute kinds that should be printed exclusively.
   /// Empty means allow all.
@@ -287,6 +288,7 @@ struct PrintOptions {
   static PrintOptions printEverything() {
     PrintOptions result = printVerbose();
     result.ExcludeAttrList.clear();
+    result.ExcludeAttrList.push_back(DAK_FixedLayout);
     result.PrintStorageRepresentationAttrs = true;
     result.AbstractAccessors = false;
     result.PrintAccessibility = true;

lib/AST/Decl.cpp

@@ -1857,6 +1857,10 @@ bool NominalTypeDecl::hasFixedLayout() const {
     return true;
   }
 
+  // Objective-C enums always have a fixed layout.
+  if (isa<EnumDecl>(this) && isObjC())
+    return true;
+
   // Otherwise, access via indirect "resilient" interfaces.
   return false;
 }

lib/Sema/TypeCheckDecl.cpp

@@ -6957,18 +6957,17 @@ static void validateFixedLayoutAttribute(TypeChecker &TC,
                                          NominalTypeDecl *D) {
   DeclAttributes &Attrs = D->getAttrs();
 
-  if (Attrs.hasAttribute<FixedLayoutAttr>())
+  // FIXME: Add a per-module serialized HasFixedLayout flag, instead of
+  // giving every decl this attribute.
+
+  if (Attrs.hasAttribute<FixedLayoutAttr>() ||
+      TC.Context.LangOpts.EnableResilience)
     return;
 
   // Since -enable-resilience should not change how we call into
   // existing compiled modules, make all value types @_fixed_layout
   // when the frontend is not run with the -enable-resilience flag.
-  if (!TC.Context.LangOpts.EnableResilience &&
-      D->getFormalAccess() == Accessibility::Public)
-    Attrs.add(new (TC.Context) FixedLayoutAttr(/*IsImplicit*/ true));
-  // @objc enums are always @_fixed_layout.
-  else if (isa<EnumDecl>(D) && D->isObjC())
-    Attrs.add(new (TC.Context) FixedLayoutAttr(/*IsImplicit*/ true));
+  Attrs.add(new (TC.Context) FixedLayoutAttr(/*IsImplicit*/ true));
 }
 
 static void validateAttributes(TypeChecker &TC, Decl *D) {