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_symbol_mangling/src/v0.rs

@@ -427,7 +427,8 @@ impl<'tcx> Printer<'tcx> for SymbolMangler<'tcx> {
                 self.print_def_path(def_id, &[])?;
             }
 
-            ty::FnPtr(sig) => {
+            ty::FnPtr(sig_tys, hdr) => {
+                let sig = sig_tys.with(hdr);
                 self.push("F");
                 self.in_binder(&sig, |cx, sig| {
                     if sig.safety == hir::Safety::Unsafe {