cmd/compile: restore original assignment error messages
When used with the compiler, types2 will report assignment error
messages that closely match what the compiler type checker (types1)
produces.
Also, mark lhs variables as used in invalid variable initializations
to avoid a class of follow-on errors.
Fixes #48558.
Change-Id: I92d1de006c66b3a2364bb1bea773a312963afe75
Reviewed-on: https://go-review.googlesource.com/c/go/+/351669
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org> Reviewed-by: Robert Findley <rfindley@google.com>