[dev.typeparams] cmd/compile/internal/types2: overlapping embedded interfaces requires go1.14

Add respective check to type checker.
Enables another excluded test in test/run.go.

This CL completes the currently required checks for
language compatibility in types2.

Updates #31793.

Change-Id: Icececff9e6023d38f600c93bcb54cdcafcf501b9
Reviewed-on: https://go-review.googlesource.com/c/go/+/290911


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