[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
+```