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_borrowck/src/lib.rs

@@ -1644,7 +1644,7 @@ impl<'mir, 'tcx> MirBorrowckCtxt<'_, 'mir, '_, 'tcx> {
                     | ty::Pat(_, _)
                     | ty::Slice(_)
                     | ty::FnDef(_, _)
-                    | ty::FnPtr(_)
+                    | ty::FnPtr(..)
                     | ty::Dynamic(_, _, _)
                     | ty::Closure(_, _)
                     | ty::CoroutineClosure(_, _)
@@ -1689,7 +1689,7 @@ impl<'mir, 'tcx> MirBorrowckCtxt<'_, 'mir, '_, 'tcx> {
                     | ty::RawPtr(_, _)
                     | ty::Ref(_, _, _)
                     | ty::FnDef(_, _)
-                    | ty::FnPtr(_)
+                    | ty::FnPtr(..)
                     | ty::Dynamic(_, _, _)
                     | ty::CoroutineWitness(..)
                     | ty::Never