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_codegen_ssa/src/debuginfo/type_names.rs

@@ -331,7 +331,7 @@ fn push_debuginfo_type_name<'tcx>(
                 output.push(')');
             }
         }
-        ty::FnDef(..) | ty::FnPtr(_) => {
+        ty::FnDef(..) | ty::FnPtr(..) => {
             // We've encountered a weird 'recursive type'
             // Currently, the only way to generate such a type
             // is by using 'impl trait':