Insert null checks for pointer dereferences when debug assertions are enabled

Similar to how the alignment is already checked, this adds a check
for null pointer dereferences in debug mode. It is implemented similarly
to the alignment check as a MirPass.

This is related to a 2025H1 project goal for better UB checks in debug
mode: https://github.com/rust-lang/rust-project-goals/pull/177.

compiler/rustc_span/src/symbol.rs

@@ -1476,6 +1476,7 @@ symbols! {
         panic_location,
         panic_misaligned_pointer_dereference,
         panic_nounwind,
+        panic_null_pointer_dereference,
         panic_runtime,
         panic_str_2015,
         panic_unwind,