cmd/compile/internal/typecheck: record whether an interface is implicit

In preparation for capturing the implicit interface bit in export data,
thread through the IsImplicit property from types2 into typecheck.

Change-Id: I9b46fe73de102935a127e6ececaacd76738b557e
Reviewed-on: https://go-review.googlesource.com/c/go/+/357109


Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>