Add miri test matching on !.

Author: cjgillot

Diff for: src/tools/miri/tests/fail/never_match_never.rs

@@ -0,0 +1,10 @@
+// This should fail even without validation
+//@compile-flags: -Zmiri-disable-validation
+
+#![feature(never_type)]
+#![allow(unreachable_code)]
+
+fn main() {
+    let ptr: *const (i32, !) = &0i32 as *const i32 as *const _;
+    unsafe { match (*ptr).1 {} } //~ ERROR: entering unreachable code
+}

Diff for: src/tools/miri/tests/fail/never_match_never.stderr

@@ -0,0 +1,15 @@
+error: Undefined Behavior: entering unreachable code
+  --> $DIR/never_match_never.rs:LL:CC
+   |
+LL |     unsafe { match (*ptr).1 {} }
+   |                    ^^^^^^^^ entering unreachable code
+   |
+   = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
+   = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
+   = note: BACKTRACE:
+   = note: inside `main` at $DIR/never_match_never.rs:LL:CC
+
+note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace
+
+error: aborting due to previous error
+