Add attr to enable lexical lifetime per function.

Enables modules to migrate to the new lifetime rules incrementally.

Author: nate-chandler

gyb_syntax_support/AttributeKinds.py

@@ -603,6 +603,12 @@ def __init__(self, name, swift_name=None):
                         OnFunc,  OnParam,  OnVar,  OnNominalType,
                         code=117),
 
+    SimpleDeclAttribute('_lexicalLifetimes', 'LexicalLifetimes',
+                        UserInaccessible,
+                        ABIStableToAdd,  ABIStableToRemove,
+                        APIStableToAdd,  APIStableToRemove,
+                        OnFunc, code=36),
+
     SimpleDeclAttribute('_noEagerMove', 'NoEagerMove',
                         UserInaccessible,
                         ABIStableToAdd,  ABIStableToRemove,