Kill [stdlib] attribute.

Now that we have true serialized modules, the standard library can import
the Builtin module without any special direction (beyond -parse-stdlib),
and anyone can include those modules without special direction.

Swift SVN r6752

include/swift/Serialization/SerializedModuleLoader.h

@@ -54,8 +54,7 @@ public:
   /// \returns the module referenced, if it could be loaded. Otherwise,
   /// emits a diagnostic and returns NULL.
   virtual Module *
-  loadModule(SourceLoc importLoc, Module::AccessPathTy path,
-             bool isStdlibImport) override;
+  loadModule(SourceLoc importLoc, Module::AccessPathTy path) override;
 
   /// \brief Look for declarations associated with the given name.
   ///