This reverts CL 151157.

CL 151157 introduced a crash when decoding into ",string" fields. It
came with a moderate speedup, so at this stage of the release cycle
let's just revert it, and reapply it in Go 1.14 with the fix in CL 190659.

Also applied the test cases from CL 190659.

Updates #33728

Change-Id: Ie46e2bc15224b251888580daf6b79d5865f3878e
Reviewed-on: https://go-review.googlesource.com/c/go/+/190909
Run-TryBot: Andrew Bonventre <andybons@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Andrew Bonventre <andybons@golang.org>