c-variadic: check that c-variadic functions cannot be tail-called

as far as I can see this was not tested, though the error message was already implemented

tests/ui/explicit-tail-calls/c-variadic.rs

@@ -0,0 +1,14 @@
+#![expect(incomplete_features)]
+#![feature(c_variadic, explicit_tail_calls)]
+#![allow(unused)]
+
+unsafe extern "C" fn foo(mut ap: ...) -> u32 {
+    ap.arg::<u32>()
+}
+
+extern "C" fn bar() -> u32 {
+    unsafe { become foo(1, 2, 3) }
+    //~^ ERROR c-variadic functions can't be tail-called
+}
+
+fn main() {}

tests/ui/explicit-tail-calls/c-variadic.stderr

@@ -0,0 +1,8 @@
+error: c-variadic functions can't be tail-called
+  --> $DIR/c-variadic.rs:10:14
+   |
+LL |     unsafe { become foo(1, 2, 3) }
+   |              ^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 1 previous error
+