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/librustdoc/clean/mod.rs

@@ -32,7 +32,7 @@ use rustc::middle::ty;
 
 use std::rc::Rc;
 use std::u32;
-use std::gc::Gc;
+use std::gc::{Gc, GC};
 
 use core;
 use doctree;

src/librustdoc/core.rs

@@ -18,6 +18,7 @@ use syntax::parse::token;
 use syntax;
 
 use std::cell::RefCell;
+use std::gc::GC;
 use std::os;
 use std::collections::{HashMap, HashSet};
 

src/librustdoc/test.rs

@@ -10,12 +10,13 @@
 
 use std::cell::RefCell;
 use std::char;
-use std::io;
+use std::dynamic_lib::DynamicLibrary;
+use std::gc::GC;
 use std::io::{Command, TempDir};
+use std::io;
 use std::os;
 use std::str;
 use std::string::String;
-use std::dynamic_lib::DynamicLibrary;
 
 use std::collections::{HashSet, HashMap};
 use testing;

src/librustdoc/visit_ast.rs

@@ -18,7 +18,7 @@ use syntax::ast_map;
 use syntax::attr::AttrMetaMethods;
 use syntax::codemap::Span;
 
-use std::gc::Gc;
+use std::gc::{Gc, GC};
 
 use core;
 use doctree::*;