Add regression test for rust-lang#65394

Author: ecstatic-morse

src/test/ui/consts/const-eval/issue-65394.rs

@@ -0,0 +1,13 @@
+// Test for absence of validation mismatch ICE in #65394
+
+#![feature(rustc_attrs)]
+
+#[rustc_mir(borrowck_graphviz_postflow="hello.dot")]
+const _: Vec<i32> = {
+    let mut x = Vec::<i32>::new();
+    let r = &mut x; //~ ERROR references in constants may only refer to immutable values
+    let y = x;
+    y
+};
+
+fn main() {}

src/test/ui/consts/const-eval/issue-65394.stderr

@@ -0,0 +1,11 @@
+error[E0017]: references in constants may only refer to immutable values
+  --> $DIR/issue-65394.rs:8:13
+   |
+LL |     let r = &mut x;
+   |             ^^^^^^ constants require immutable values
+
+[ERROR rustc_mir::transform::qualify_consts] old validator: [($DIR/issue-65394.rs:8:13: 8:19, "MutBorrow(Mut { allow_two_phase_borrow: false })")]
+[ERROR rustc_mir::transform::qualify_consts] new validator: [($DIR/issue-65394.rs:8:13: 8:19, "MutBorrow(Mut { allow_two_phase_borrow: false })"), ($DIR/issue-65394.rs:7:9: 7:14, "LiveDrop")]
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0017`.