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_trait_selection/src/traits/project.rs

@@ -1636,7 +1636,7 @@ fn confirm_fn_pointer_candidate<'cx, 'tcx>(
             .generics_of(def_id)
             .host_effect_index
             .map_or(tcx.consts.true_, |idx| args.const_at(idx)),
-        ty::FnPtr(_) => tcx.consts.true_,
+        ty::FnPtr(..) => tcx.consts.true_,
         _ => unreachable!("only expected FnPtr or FnDef in `confirm_fn_pointer_candidate`"),
     };