text/template, html/template: use reflect.TypeFor for known types

text/template, html/template: use reflect.TypeFor for known types

For #60088

Change-Id: Ibc3983ca5cfe396087ddfa96c43cfe32ca47129a
Reviewed-on: https://go-review.googlesource.com/c/go/+/514640
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>