Use if-let guards in the codebase

compiler/rustc_span/src/lib.rs

@@ -16,10 +16,12 @@
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
 #![feature(array_windows)]
 #![feature(crate_visibility_modifier)]
+#![feature(if_let_guard)]
 #![feature(negative_impls)]
 #![feature(nll)]
 #![feature(min_specialization)]
 #![feature(thread_local_const_init)]
+#![cfg_attr(bootstrap, allow(incomplete_features))] // if_let_guard
 
 #[macro_use]
 extern crate rustc_macros;