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_hir_analysis/src/check/wfcheck.rs

@@ -951,7 +951,7 @@ fn check_param_wf(tcx: TyCtxt<'_>, param: &hir::GenericParam<'_>) -> Result<(),
             } else {
                 let mut diag = match ty.kind() {
                     ty::Bool | ty::Char | ty::Int(_) | ty::Uint(_) | ty::Error(_) => return Ok(()),
-                    ty::FnPtr(_) => tcx.dcx().struct_span_err(
+                    ty::FnPtr(..) => tcx.dcx().struct_span_err(
                         hir_ty.span,
                         "using function pointers as const generic parameters is forbidden",
                     ),