[ty] Expansion of enums into unions of literals (#19382)

## Summary

Implement expansion of enums into unions of enum literals (and the
reverse operation). For the enum below, this allows us to understand
that `Color = Literal[Color.RED, Color.GREEN, Color.BLUE]`, or that
`Color & ~Literal[Color.RED] = Literal[Color.GREEN, Color.BLUE]`. This
helps in exhaustiveness checking, which is why we see some removed
`assert_never` false positives. And since exhaustiveness checking also
helps with understanding terminal control flow, we also see a few
removed `invalid-return-type` and `possibly-unresolved-reference` false
positives. This PR also adds expansion of enums in overload resolution
and type narrowing constructs.

```py
from enum import Enum
from typing_extensions import Literal, assert_never
from ty_extensions import Intersection, Not, static_assert, is_equivalent_to

class Color(Enum):
    RED = 1
    GREEN = 2
    BLUE = 3

type Red = Literal[Color.RED]
type Green = Literal[Color.GREEN]
type Blue = Literal[Color.BLUE]

static_assert(is_equivalent_to(Red | Green | Blue, Color))
static_assert(is_equivalent_to(Intersection[Color, Not[Red]], Green | Blue))


def color_name(color: Color) -> str:  # no error here (we detect that this can not implicitly return None)
    if color is Color.RED:
        return "Red"
    elif color is Color.GREEN:
        return "Green"
    elif color is Color.BLUE:
        return "Blue"
    else:
        assert_never(color)  # no error here
```

## Performance

I avoided an initial regression here for large enums, but the
`UnionBuilder` and `IntersectionBuilder` parts can certainly still be
optimized. We might want to use the same technique that we also use for
unions of other literals. I didn't see any problems in our benchmarks so
far, so this is not included yet.

## Test Plan

Many new Markdown tests

crates/ty_python_semantic/resources/mdtest/narrow/conditionals/eq.md

@@ -1,4 +1,4 @@
-# Narrowing for `!=` conditionals
+# Narrowing for `!=` and `==` conditionals
 
 ## `x != None`
 
@@ -22,6 +22,12 @@ def _(x: bool):
         reveal_type(x)  # revealed: Literal[True]
     else:
         reveal_type(x)  # revealed: Literal[False]
+
+def _(x: bool):
+    if x == False:
+        reveal_type(x)  # revealed: Literal[False]
+    else:
+        reveal_type(x)  # revealed: Literal[True]
 ```
 
 ### Enums
@@ -35,11 +41,31 @@ class Answer(Enum):
 
 def _(answer: Answer):
     if answer != Answer.NO:
-        # TODO: This should be simplified to `Literal[Answer.YES]`
-        reveal_type(answer)  # revealed: Answer & ~Literal[Answer.NO]
+        reveal_type(answer)  # revealed: Literal[Answer.YES]
     else:
-        # TODO: This should be `Literal[Answer.NO]`
-        reveal_type(answer)  # revealed: Answer
+        reveal_type(answer)  # revealed: Literal[Answer.NO]
+
+def _(answer: Answer):
+    if answer == Answer.NO:
+        reveal_type(answer)  # revealed: Literal[Answer.NO]
+    else:
+        reveal_type(answer)  # revealed: Literal[Answer.YES]
+
+class Single(Enum):
+    VALUE = 1
+
+def _(x: Single | int):
+    if x != Single.VALUE:
+        reveal_type(x)  # revealed: int
+    else:
+        # `int` is not eliminated here because there could be subclasses of `int` with custom `__eq__`/`__ne__` methods
+        reveal_type(x)  # revealed: Single | int
+
+def _(x: Single | int):
+    if x == Single.VALUE:
+        reveal_type(x)  # revealed: Single | int
+    else:
+        reveal_type(x)  # revealed: int
 ```
 
 This narrowing behavior is only safe if the enum has no custom `__eq__`/`__ne__` method:

crates/ty_python_semantic/resources/mdtest/narrow/conditionals/is.md

@@ -78,8 +78,16 @@ def _(answer: Answer):
     if answer is Answer.NO:
         reveal_type(answer)  # revealed: Literal[Answer.NO]
     else:
-        # TODO: This should be `Literal[Answer.YES]`
-        reveal_type(answer)  # revealed: Answer & ~Literal[Answer.NO]
+        reveal_type(answer)  # revealed: Literal[Answer.YES]
+
+class Single(Enum):
+    VALUE = 1
+
+def _(x: Single | int):
+    if x is Single.VALUE:
+        reveal_type(x)  # revealed: Single
+    else:
+        reveal_type(x)  # revealed: int
 ```
 
 ## `is` for `EllipsisType` (Python 3.10+)

crates/ty_python_semantic/resources/mdtest/narrow/conditionals/is_not.md

@@ -18,6 +18,8 @@ def _(flag: bool):
 
 ## `is not` for other singleton types
 
+Boolean literals:
+
 ```py
 def _(flag: bool):
     x = True if flag else False
@@ -29,6 +31,33 @@ def _(flag: bool):
         reveal_type(x)  # revealed: Literal[False]
 ```
 
+Enum literals:
+
+```py
+from enum import Enum
+
+class Answer(Enum):
+    NO = 0
+    YES = 1
+
+def _(answer: Answer):
+    if answer is not Answer.NO:
+        reveal_type(answer)  # revealed: Literal[Answer.YES]
+    else:
+        reveal_type(answer)  # revealed: Literal[Answer.NO]
+
+    reveal_type(answer)  # revealed: Answer
+
+class Single(Enum):
+    VALUE = 1
+
+def _(x: Single | int):
+    if x is not Single.VALUE:
+        reveal_type(x)  # revealed: int
+    else:
+        reveal_type(x)  # revealed: Single
+```
+
 ## `is not` for non-singleton types
 
 Non-singleton types should *not* narrow the type: two instances of a non-singleton class may occupy