[dev.typeparams] cmd/compile: fixing import of comm clauses/closures in generic functions

Improvements:
 - Fix export/import of the default case of a select statement (was not
   dealing with nil Comm case)
 - Set properly the name of closure functions in imported generic
   functions

Added new test exporting/importing a reasonably large channel package,
chansimp.go.

Change-Id: If2ee12bd749e5df415f48ec4b629a2fa68a79dcb
Reviewed-on: https://go-review.googlesource.com/c/go/+/321734


Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Trust: Dan Scales <danscales@google.com>