cmd/compile/internal/types2: better error reporting framework (starting point)

Until now, errors which came with additional details (e.g., a declaration
cycle error followed by the list of objects involved in the cycle, one per
line) were reported as an ordinary error followed by "secondary" errors,
with the secondary errors marked as such by having a tab-indented error
message.

This approach often required clients to filter these secondary errors
(as they are not new errors, they are just clarifying a previously
reported error).

This CL introduces a new internal error_ type which permits accumulating
various error information that may then be reported as a single error.

Change-Id: I25b2f094facd37e12737e517f7ef8853d465ff77
Reviewed-on: https://go-review.googlesource.com/c/go/+/296689


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>