syntax: Make quasiquoter use absolute paths

As part of removing `pub use` glob, two extra import globs were
injected to make `quote_expr!` work. However the globs caused
`unused_import` warning in some places.

Quasiquoter needed the globs since it generated idents (e.g. `TyU`)
rather than absolute paths (`::syntax::ast::TyU`).
This patch removes the extra globs and makes quasiquoter use absolute
paths.

Fixes #14618

src/libregex_macros/lib.rs

@@ -20,7 +20,6 @@
        html_root_url = "http://doc.rust-lang.org/")]
 
 #![feature(macro_registrar, managed_boxes, quote)]
-#![allow(unused_imports)] // `quote_expr!` adds some `use` globs which may be unused
 
 extern crate regex;
 extern crate syntax;