cmd/compile: fixing 15.go for -G=3

cmd/compile: fixing 15.go for -G=3

Required two changes:

 - avoid creating a closure in the case where the actual receiver of an
   embedded method is not generic even though the base operand of the
   selector is generic. This is similar to the test suggested by wayne
   zuo - I thought it was clear in buildClosure, and easier to comment.

 - Propagate //go:nointerface to base generic methods and then to
   instantiations.

Change-Id: If30c834e4223c2639b7f7e74d44e6087aa9ccd76
Reviewed-on: https://go-review.googlesource.com/c/go/+/344251
Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Wayne Zuo <wdvxdr1123@gmail.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Trust: Dan Scales <danscales@google.com>