cmd/go: prepend builtin prolog when checking for preamble errors
Fixes #50710
Change-Id: I62feddbe3eaae9605d196bec60d378614436603a
Reviewed-on: https://go-review.googlesource.com/c/go/+/379754
Auto-Submit: Ian Lance Taylor <iant@google.com> Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Ian Lance Taylor <iant@google.com> Reviewed-by: Ian Lance Taylor <iant@google.com> Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>