[red-knot] Infer type of if-expression if test has statically known truthiness (#14048)

## Summary

Detecting statically known truthy or falsy test in if expressions
(ternary).

## Test Plan

new mdtest

crates/red_knot_python_semantic/resources/mdtest/expression/if.md

@@ -0,0 +1,24 @@
+# If expression
+
+## Union
+
+```py
+def bool_instance() -> bool:
+    return True
+
+reveal_type(1 if bool_instance() else 2)  # revealed: Literal[1, 2]
+```
+
+## Statically known branches
+
+```py
+reveal_type(1 if True else 2)  # revealed: Literal[1]
+reveal_type(1 if "not empty" else 2)  # revealed: Literal[1]
+reveal_type(1 if (1,) else 2)  # revealed: Literal[1]
+reveal_type(1 if 1 else 2)  # revealed: Literal[1]
+
+reveal_type(1 if False else 2)  # revealed: Literal[2]
+reveal_type(1 if None else 2)  # revealed: Literal[2]
+reveal_type(1 if "" else 2)  # revealed: Literal[2]
+reveal_type(1 if 0 else 2)  # revealed: Literal[2]
+```

crates/red_knot_python_semantic/resources/mdtest/subscript/class.md

@@ -39,7 +39,8 @@ reveal_type(UnionClassGetItem[0])  # revealed: str | int
 ## Class getitem with class union
 
 ```py
-flag = True
+def bool_instance() -> bool:
+    return True
 
 class A:
     def __class_getitem__(cls, item: int) -> str:
@@ -49,7 +50,7 @@ class B:
     def __class_getitem__(cls, item: int) -> int:
         return item
 
-x = A if flag else B
+x = A if bool_instance() else B
 
 reveal_type(x)  # revealed: Literal[A, B]
 reveal_type(x[0])  # revealed: str | int

crates/red_knot_python_semantic/src/types/infer.rs

@@ -2362,13 +2362,15 @@ impl<'db> TypeInferenceBuilder<'db> {
             orelse,
         } = if_expression;
 
-        self.infer_expression(test);
-
-        // TODO detect statically known truthy or falsy test
+        let test_ty = self.infer_expression(test);
         let body_ty = self.infer_expression(body);
         let orelse_ty = self.infer_expression(orelse);
 
-        UnionType::from_elements(self.db, [body_ty, orelse_ty])
+        match test_ty.bool(self.db) {
+            Truthiness::AlwaysTrue => body_ty,
+            Truthiness::AlwaysFalse => orelse_ty,
+            Truthiness::Ambiguous => UnionType::from_elements(self.db, [body_ty, orelse_ty]),
+        }
     }
 
     fn infer_lambda_body(&mut self, lambda_expression: &ast::ExprLambda) {