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/hygiene.rs

@@ -1330,7 +1330,7 @@ fn update_disambiguator(expn_id: ExpnId) {
             // This cache is only used by `DummyHashStableContext`,
             // so we won't pollute the cache values of the normal `StableHashingContext`
             thread_local! {
-                static CACHE: ExpnIdCache = Default::default();
+                static CACHE: ExpnIdCache = const { ExpnIdCache::new(Vec::new()) };
             }
 
             &CACHE

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;