Handle panicking like rustc CTFE does

Instead of using `core::fmt::format` to format panic messages, which may in turn
panic too and cause recursive panics and other messy things, redirect
`panic_fmt` to `const_panic_fmt` like CTFE, which in turn goes to
`panic_display` and does the things normally. See the tests for the full
call stack.

crates/ide/src/hover/tests.rs

@@ -5105,6 +5105,32 @@ fn foo(e: E) {
     );
 }
 
+#[test]
+fn hover_const_value() {
+    check(
+        r#"
+pub enum AA {
+    BB,
+}
+const CONST: AA = AA::BB;
+pub fn the_function() -> AA {
+    CON$0ST
+}
+"#,
+        expect![[r#"
+            *CONST*
+
+            ```rust
+            test
+            ```
+
+            ```rust
+            const CONST: AA = BB
+            ```
+        "#]],
+    );
+}
+
 #[test]
 fn array_repeat_exp() {
     check(