Part of the general trend of moving yyerror calls out of walk and into
typecheck.

Notably, this requires splitting test/typeswitch2.go into two files,
because now some of the errors are reported during typecheck and
others are still reported during walk; and if there were any errors
during typecheck, then cmd/compile exits without invoking walk.

Passes toolstash-check.

Change-Id: I05ee0c00b99af659ee1eef098d342d0d736cf31e
Reviewed-on: https://go-review.googlesource.com/c/go/+/194659Reviewed-by: Robert Griesemer <gri@golang.org>