Narrowing types to `Literal` using `in` syntax

Take this example code:

x = "a"
reveal_type(x)
assert x in ("a", "b", "c")
reveal_type(x)

y = 20
reveal_type(y)
assert y == 10 or y == 20 or y == 50
reveal_type(y)

The current output of this is:

$ mypy x.py 
x.py:2: note: Revealed type is "builtins.str"
x.py:4: note: Revealed type is "builtins.str"
x.py:7: note: Revealed type is "builtins.int"
x.py:9: note: Revealed type is "builtins.int"

If mypy were to support type narrowing using chained or statements and in statements, the output would look like:

$ mypy x.py 
x.py:4: note: Revealed type is "builtins.str"
x.py:6: note: Revealed type is "Union[Literal['a'], Literal['b'], Literal['c']]"
x.py:9: note: Revealed type is "builtins.int"
x.py:11: note: Revealed type is "Union[Literal[10], Literal[20], Literal[50]]"

I think doing simple narrowing here would be relatively easy and very useful.

Start with the x.py examples in the issue and run mypy to confirm the current revealed types. Investigate mypy's existing type-narrowing behavior for chained or conditions and in expressions; done means the examples reveal the requested Literal unions for both string and integer cases.