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/ext/build.rs

@@ -21,7 +21,7 @@ use parse::token::special_idents;
 use parse::token::InternedString;
 use parse::token;
 
-use std::gc::Gc;
+use std::gc::{Gc, GC};
 
 // Transitional reexports so qquote can find the paths it is looking for
 mod syntax {