[dev.typeparams] merge master into dev.typeparams

On top of the merge, the following fixes were applied:
 + Debug["G"] changed to Debug.G, following golang.org/cl/263539.
 + issue42058a.go and issue42058b.go were skipped in
   types2/stdlib_test.go. go/types does not produce errors related to
   channel element size.

Change-Id: I59fc84e12a2d728ef789fdc616f7afe80e451283