go/types, types2: better error messages for channel sends and receives
Use the same code pattern for sends and receives and factor it out
into a new helper method Checker.chanElem.
Provide the exact error cause rather than simply referring to the
core type.
For #70128.
Change-Id: I4a0b597a487b78c057eebe06c4ac28f9bf1f7719
Reviewed-on: https://go-review.googlesource.com/c/go/+/647455
Auto-Submit: Robert Griesemer <gri@google.com> Reviewed-by: Robert Findley <rfindley@google.com> Reviewed-by: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>