Shrink TyKind::FnPtr.

By splitting the `FnSig` within `TyKind::FnPtr` into `FnSigTys` and
`FnHeader`, which can be packed more efficiently. This reduces the size
of the hot `TyKind` type from 32 bytes to 24 bytes on 64-bit platforms.
This reduces peak memory usage by a few percent on some benchmarks. It
also reduces cache misses and page faults similarly, though this doesn't
translate to clear cycles or wall-time improvements on CI.

compiler/rustc_ty_utils/src/instance.rs

@@ -248,7 +248,7 @@ fn resolve_associated_item<'tcx>(
                 if name == sym::clone {
                     let self_ty = trait_ref.self_ty();
                     match self_ty.kind() {
-                        ty::FnDef(..) | ty::FnPtr(_) => (),
+                        ty::FnDef(..) | ty::FnPtr(..) => (),
                         ty::Coroutine(..)
                         | ty::CoroutineWitness(..)
                         | ty::Closure(..)