Correctly escape strings in our quote macro

This is a small change, but it was the cause of 90% of the errors in `rust-analyzer diagnostics .` 🫢

With this change and #18085 together, all remaining errors are type errors.

This may mean we can enable more errors, but this is out of scope for this PR.

crates/hir-def/src/macro_expansion_tests/builtin_fn_macro.rs

@@ -528,3 +528,21 @@ fn main() { foobar; }
 "##]],
     );
 }
+
+#[test]
+fn test_quote_string() {
+    check(
+        r##"
+#[rustc_builtin_macro]
+macro_rules! stringify {}
+
+fn main() { stringify!("hello"); }
+"##,
+        expect![[r##"
+#[rustc_builtin_macro]
+macro_rules! stringify {}
+
+fn main() { "\"hello\""; }
+"##]],
+    );
+}