Fix python#11971

Allows PlaceholderNode objects before raising an error for the attempted
reuse of a member name inside an Enum.

mypy/semanal.py

@@ -2871,7 +2871,7 @@ def analyze_name_lvalue(self,
         outer = self.is_global_or_nonlocal(name)
         if kind == MDEF and isinstance(self.type, TypeInfo) and self.type.is_enum:
             # Special case: we need to be sure that `Enum` keys are unique.
-            if existing:
+            if existing is not None and not isinstance(existing.node, PlaceholderNode):
                 self.fail('Attempted to reuse member name "{}" in Enum definition "{}"'.format(
                     name, self.type.name,
                 ), lvalue)