Sema: Targeted fix for LLDB REPL issue

LLDB changes accessiblity of declarations after type checking,
which is not a good idea because it is likely to break invariants.
Indeed, the validateFixedLayoutAttribute() / hasFixedLayout()
logic was not prepared for this possibility.

This is a targeted fix to address the immediate breakage. A better
fix would be to change LLDB, and also to change Sema to store the
global -enable-resilience flag state in a bit in the serialized
module, instead of sticking it on every declaration.

Fixes <rdar://problem/23545959>.

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;
 }