add const eval bool-to-int cast test

Author: RalfJung

tests/ui/consts/const-eval/ub-invalid-values.rs

@@ -0,0 +1,11 @@
+const fn bool_cast(ptr: *const bool) { unsafe {
+    let _val = *ptr as u32; //~ERROR: evaluation of constant value failed
+    //~^ interpreting an invalid 8-bit value as a bool
+}}
+
+const _: () = {
+    let v = 3_u8;
+    bool_cast(&v as *const u8 as *const bool);
+};
+
+fn main() {}

tests/ui/consts/const-eval/ub-invalid-values.stderr

@@ -0,0 +1,20 @@
+error[E0080]: evaluation of constant value failed
+  --> $DIR/ub-invalid-values.rs:2:16
+   |
+LL |     let _val = *ptr as u32;
+   |                ^^^^^^^^^^^ interpreting an invalid 8-bit value as a bool: 0x03
+   |
+note: inside `bool_cast`
+  --> $DIR/ub-invalid-values.rs:2:16
+   |
+LL |     let _val = *ptr as u32;
+   |                ^^^^^^^^^^^
+note: inside `_`
+  --> $DIR/ub-invalid-values.rs:8:5
+   |
+LL |     bool_cast(&v as *const u8 as *const bool);
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0080`.