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_util.rs

@@ -21,7 +21,7 @@ use visit;
 
 use std::cell::Cell;
 use std::cmp;
-use std::gc::Gc;
+use std::gc::{Gc, GC};
 use std::u32;
 
 pub fn path_name_i(idents: &[Ident]) -> String {