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/errors.rs

@@ -217,3 +217,7 @@ pub(crate) struct MismatchedDataLayout<'a> {
 pub(crate) struct FixedX18InvalidArch<'a> {
     pub arch: &'a str,
 }
+
+#[derive(Diagnostic)]
+#[diag(codegen_llvm_sanitizer_kcfi_arity_requires_llvm_21_0_0)]
+pub(crate) struct SanitizerKcfiArityRequiresLLVM2100;