add known-bug test for unsoundness issue rust-lang#107975

Part of the resolution to rust-lang#105107

Author: gburgessiv

tests/ui/codegen/issue-107975-pointer-inequality.rs

@@ -0,0 +1,17 @@
+// run-pass
+// known-bug: #107975
+fn main() {
+    let a: *const u8;
+    let b: *const u8;
+    {
+        let v: [u8; 16] = [core::hint::black_box(0); 16];
+        a = &(v[0]);
+    }
+    {
+        let v: [u8; 16] = [core::hint::black_box(0); 16];
+        b = &(v[0]);
+    }
+    assert_ne!(a, b);
+    println!("{a:?} {b:?}");
+    assert_eq!(a, b);
+}