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/libsyntax/ast.rs

@@ -1804,7 +1804,7 @@ impl Arg {
 pub struct FnDecl {
     pub inputs: Vec<Arg>,
     pub output: FunctionRetTy,
-    pub variadic: bool,
+    pub c_variadic: bool,
 }
 
 impl FnDecl {