Add FIXME note to const generics test

Co-authored-by: lcnr <bastian_kauschke@hotmail.de>

src/test/ui/issues/issue-75763.rs

@@ -8,6 +8,7 @@ struct Bug<const S: &'static str>;
 fn main() {
     let b: Bug::<{
         unsafe {
+            // FIXME(const_generics): Decide on how to deal with invalid values as const params.
             std::mem::transmute::<&[u8], &str>(&[0xC0, 0xC1, 0xF5])
         }
     }>;