[red-knot] Do not panic when encountering string annotations (#14091)

## Summary

Encountered this while running red-knot benchmarks on the `black`
codebase.

Fixes two of the issues in #13478.

## Test Plan

Added a regression test.

crates/red_knot_python_semantic/resources/mdtest/annotations/string_annotations.md

@@ -0,0 +1,9 @@
+# String annotations
+
+```py
+def f() -> "int":
+    return 1
+
+# TODO: We do not support string annotations, but we should not panic if we encounter them
+reveal_type(f())  # revealed: @Todo
+```

crates/red_knot_python_semantic/src/types/infer.rs

@@ -3725,7 +3725,10 @@ impl<'db> TypeInferenceBuilder<'db> {
             // TODO: parse the expression and check whether it is a string annotation, since they
             // can be annotation expressions distinct from type expressions.
             // https://typing.readthedocs.io/en/latest/spec/annotations.html#string-annotations
-            ast::Expr::StringLiteral(_literal) => Type::Todo,
+            ast::Expr::StringLiteral(_literal) => {
+                self.store_expression_type(expression, Type::Todo);
+                Type::Todo
+            }
 
             // Annotation expressions also get special handling for `*args` and `**kwargs`.
             ast::Expr::Starred(starred) => self.infer_starred_expression(starred),