rustc: Obsolete the @ syntax entirely

This removes all remnants of `@` pointers from rustc. Additionally, this removes
the `GC` structure from the prelude as it seems odd exporting an experimental
type in the prelude by default.

Closes #14193
[breaking-change]

src/libsyntax/ast.rs

@@ -21,7 +21,7 @@ use std::fmt;
 use std::fmt::Show;
 use std::option::Option;
 use std::rc::Rc;
-use std::gc::Gc;
+use std::gc::{Gc, GC};
 use serialize::{Encodable, Decodable, Encoder, Decoder};
 
 /// A pointer abstraction. FIXME(eddyb) #10676 use Rc<T> in the future.