add regression test for #85907

Author: TaKO8Ki

src/test/ui/consts/const-eval/issue-85907.rs

@@ -0,0 +1,7 @@
+const fn hey() -> usize {
+    panic!(123); //~ ERROR argument to `panic!()` in a const context must have type `&str`
+}
+
+fn main() {
+    let _: [u8; hey()] = todo!();
+}

src/test/ui/consts/const-eval/issue-85907.stderr

@@ -0,0 +1,10 @@
+error: argument to `panic!()` in a const context must have type `&str`
+  --> $DIR/issue-85907.rs:2:5
+   |
+LL |     panic!(123);
+   |     ^^^^^^^^^^^
+   |
+   = note: this error originates in the macro `$crate::panic::panic_2015` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: aborting due to previous error
+