Don't special-case class instances in binary expression inference (#15161)

Just like in #15045 for unary expressions: In binary expressions, we
were only looking for dunder expressions for `Type::Instance` types. We
had some special cases for coercing the various `Literal` types into
their corresponding `Instance` types before doing the lookup. But we can
side-step all of that by using the existing `Type::to_meta_type` and
`Type::to_instance` methods.

crates/red_knot_python_semantic/resources/mdtest/binary/classes.md

@@ -0,0 +1,27 @@
+# Binary operations on classes
+
+## Union of two classes
+
+Unioning two classes via the `|` operator is only available in Python 3.10 and later.
+
+```toml
+[environment]
+python-version = "3.10"
+```
+
+```py
+class A: ...
+class B: ...
+
+reveal_type(A | B)  # revealed: UnionType
+```
+
+## Union of two classes (prior to 3.10)
+
+```py
+class A: ...
+class B: ...
+
+# error: "Operator `|` is unsupported between objects of type `Literal[A]` and `Literal[B]`"
+reveal_type(A | B)  # revealed: Unknown
+```