KCFI: Add KCFI arity indicator support

Adds KCFI arity indicator support to the Rust compiler (see rust-lang/rust#138311,
https://github.com/llvm/llvm-project/pull/121070, and
https://lore.kernel.org/lkml/CANiq72=3ghFxy8E=AU9p+0imFxKr5iU3sd0hVUXed5BA+KjdNQ@mail.gmail.com/).

compiler/rustc_codegen_llvm/src/context.rs

@@ -327,6 +327,22 @@ pub(crate) unsafe fn create_module<'ll>(
                 pfe.prefix().into(),
             );
         }
+
+        // Add "kcfi-arity" module flag if KCFI arity indicator is enabled. (See
+        // https://github.com/llvm/llvm-project/pull/117121.)
+        if sess.is_sanitizer_kcfi_arity_enabled() {
+            // KCFI arity indicator requires LLVM 21.0.0 or later.
+            if llvm_version < (21, 0, 0) {
+                tcx.dcx().emit_err(crate::errors::SanitizerKcfiArityRequiresLLVM2100);
+            }
+
+            llvm::add_module_flag_u32(
+                llmod,
+                llvm::ModuleFlagMergeBehavior::Override,
+                "kcfi-arity",
+                1,
+            );
+        }
     }
 
     // Control Flow Guard is currently only supported by MSVC and LLVM on Windows.