Use new thread-local const-init

Let's see if this gives us any speedup - some of the TLS state modified in this
commit *is* pretty heavily accessed, so we can hope!

compiler/rustc_span/src/lib.rs

@@ -20,6 +20,7 @@
 #![feature(negative_impls)]
 #![feature(nll)]
 #![feature(min_specialization)]
+#![feature(thread_local_const_init)]
 
 #[macro_use]
 extern crate rustc_macros;