net/rpc: use reflect.TypeFor for known types

net/rpc: use reflect.TypeFor for known types

For #60088

Change-Id: I56586b68d5e38a46560f4ced19214f1d2db2850e
Reviewed-on: https://go-review.googlesource.com/c/go/+/514995
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Rob Pike <r@golang.org>
Auto-Submit: Ian Lance Taylor <iant@google.com>