[ty] Silence false-positive diagnostics when using typing.Dict or typing.Callable as the second argument to isinstance() (#21386)

crates/ty_python_semantic/resources/mdtest/call/builtins.md

@@ -162,3 +162,38 @@ def _(x: A | B, y: list[int]):
         reveal_type(x)  # revealed: B & ~A
         reveal_type(isinstance(x, B))  # revealed: Literal[True]
 ```
+
+Certain special forms in the typing module are not instances of `type`, so are strictly-speaking
+disallowed as the second argument to `isinstance()` according to typeshed's annotations. However, at
+runtime they work fine as the second argument, and we implement that special case in ty:
+
+```py
+import typing as t
+
+# no errors emitted for any of these:
+isinstance("", t.Dict)
+isinstance("", t.List)
+isinstance("", t.Set)
+isinstance("", t.FrozenSet)
+isinstance("", t.Tuple)
+isinstance("", t.ChainMap)
+isinstance("", t.Counter)
+isinstance("", t.Deque)
+isinstance("", t.OrderedDict)
+isinstance("", t.Callable)
+isinstance("", t.Type)
+isinstance("", t.Callable | t.Deque)
+
+# `Any` is valid in `issubclass()` calls but not `isinstance()` calls
+issubclass(list, t.Any)
+issubclass(list, t.Any | t.Dict)
+```
+
+But for other special forms that are not permitted as the second argument, we still emit an error:
+
+```py
+isinstance("", t.TypeGuard)  # error: [invalid-argument-type]
+isinstance("", t.ClassVar)  # error: [invalid-argument-type]
+isinstance("", t.Final)  # error: [invalid-argument-type]
+isinstance("", t.Any)  # error: [invalid-argument-type]
+```