Make enums idempotent

Feature

Currently, passing an enum into its own constructor forgets its value:

from enum import IntEnum

class Foo(IntEnum):
    x = 0
    y = 1

reveal_type(Foo(Foo.x))  # reveals Foo, not Foo.x
reveal_type(Foo.__new__)  # "def [Self <: IntEnum] (cls: type[Self], value: builtins.int) -> Self" 
reveal_type(Foo.__init__) # "def (self: builtins.object)"

Pitch

It should be straightforward to synthesize a signature that makes Foo(Foo.x) inferred as Foo.x.

Start by tracing enum constructor inference for IntEnum and the synthesized new and init signatures shown in the examples. Reproduce Foo(Foo.x) with reveal_type, then add coverage showing that the expression preserves the enum member value rather than only inferring Foo.