[dev.regabi] go/types: report error for invalid main function signature
This is a port of CL 279424, which didn't make it into master in time
for go1.16. Move it to dev.regabi so that it may be merged.
Notably, this port no longer removes the _InvalidInitSig error code,
instead opting to deprecate it. Now that error codes are 'locked in' for
go1.16, even if their API may not yet be exposed, we should follow the
practice of not changing their values. In the future, code generation
can make it easier to keep error code values constant.
For #43308
Change-Id: I5260b93fd063393d38d6458e45a67e7f9b7426ed
Reviewed-on: https://go-review.googlesource.com/c/go/+/289714
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org> Reviewed-by: Robert Griesemer <gri@golang.org>