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/parser/variadic-ffi-3.rs

@@ -1,5 +1,5 @@
 fn foo(x: isize, ...) {
-    //~^ ERROR: only foreign functions are allowed to be variadic
+    //~^ ERROR: only foreign functions are allowed to be C-variadic
 }
 
 fn main() {}

src/test/ui/parser/variadic-ffi-3.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/variadic-ffi-3.rs:1:18
    |
 LL | fn foo(x: isize, ...) {

src/test/ui/parser/variadic-ffi-4.rs

@@ -1,5 +1,5 @@
 extern "C" fn foo(x: isize, ...) {
-    //~^ ERROR: only foreign functions are allowed to be variadic
+    //~^ ERROR: only foreign functions are allowed to be C-variadic
 }
 
 fn main() {}

src/test/ui/parser/variadic-ffi-4.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/variadic-ffi-4.rs:1:29
    |
 LL | extern "C" fn foo(x: isize, ...) {