add test for raw_eq on a pointer

Author: RalfJung

tests/fail/intrinsics/raw_eq_on_ptr.rs

@@ -0,0 +1,11 @@
+#![feature(intrinsics)]
+
+extern "rust-intrinsic" {
+    fn raw_eq<T>(a: &T, b: &T) -> bool;
+}
+
+fn main() {
+    let x = &0;
+    // FIXME: the error message is not great (should be UB rather than 'unsupported')
+    unsafe { raw_eq(&x, &x) }; //~ERROR: unsupported operation
+}

tests/fail/intrinsics/raw_eq_on_ptr.stderr

@@ -0,0 +1,14 @@
+error: unsupported operation: unable to turn pointer into raw bytes
+  --> $DIR/raw_eq_on_ptr.rs:LL:CC
+   |
+LL |     unsafe { raw_eq(&x, &x) };
+   |              ^^^^^^^^^^^^^^ unable to turn pointer into raw bytes
+   |
+   = help: this is likely not a bug in the program; it indicates that the program performed an operation that the interpreter does not support
+   = note: backtrace:
+   = note: inside `main` at $DIR/raw_eq_on_ptr.rs:LL:CC
+
+note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace
+
+error: aborting due to previous error
+