tests: adapt for LLVM 21 changes

Per discussion in rust-lang#137799 we don't really need this readonly attribute,
so let's just drop it so the test passes on LLVM 21.

Fixes rust-lang#137799.

Author: durin42

tests/codegen/issues/issue-114312.rs

@@ -15,7 +15,6 @@ pub enum Expr {
 #[no_mangle]
 pub extern "C" fn issue_114312(expr: Expr) {
     // CHECK-LABEL: @issue_114312(
-    // CHECK-NOT: readonly
     // CHECK-SAME: byval
     // CHECK-NEXT: start:
     // CHECK-NEXT: ret void