Rollup merge of #155533 - ChayimFriedman2:const-cache-new, r=ShoyuVanilla

Provide a const new for `GlobalCache`

This can help rust-analyzer a bit.

Author: JonathanBrouwer

compiler/rustc_type_ir/src/search_graph/global_cache.rs

@@ -39,6 +39,11 @@ pub struct GlobalCache<X: Cx> {
 }
 
 impl<X: Cx> GlobalCache<X> {
+    #[inline]
+    pub const fn new() -> Self {
+        GlobalCache { map: HashMap::with_hasher(rustc_hash::FxBuildHasher) }
+    }
+
     /// Insert a final result into the global cache.
     pub(super) fn insert(
         &mut self,