Rename variadic to c_variadic

Function signatures with the `variadic` member set are actually
C-variadic functions. Make this a little more explicit by renaming the
`variadic` boolean value, `c_variadic`.

src/test/ui/invalid/invalid-variadic-function.rs

@@ -1,3 +1,3 @@
 extern "C" fn foo(x: u8, ...);
-//~^ ERROR only foreign functions are allowed to be variadic
+//~^ ERROR only foreign functions are allowed to be C-variadic
 //~| ERROR expected one of `->`, `where`, or `{`, found `;`

src/test/ui/invalid/invalid-variadic-function.stderr

@@ -1,4 +1,4 @@
-error: only foreign functions are allowed to be variadic
+error: only foreign functions are allowed to be C-variadic
   --> $DIR/invalid-variadic-function.rs:1:26
    |
 LL | extern "C" fn foo(x: u8, ...);