Add test for c_variadic in rustdoc-json

tests/rustdoc-json/fns/extern_c_variadic.rs

@@ -0,0 +1,9 @@
+#![feature(no_core)]
+#![no_core]
+
+extern "C" {
+    // @is "$.index[*][?(@.name == 'not_variadic')].inner.decl.c_variadic" false
+    pub fn not_variadic(_: i32);
+    // @is "$.index[*][?(@.name == 'variadic')].inner.decl.c_variadic" true
+    pub fn variadic(_: i32, ...);
+}