go/types, types2: add check for map value completeness in IndexExpr

go/types, types2: add check for map value completeness in IndexExpr

An index expression can also go from map[K]V to V. Since V could be
incomplete (due to some admittedly contrived syntax), we need a
check.

Change-Id: I03ffbfc0e5bcc9129591d60dfbaa5fafcf8fb183
Reviewed-on: https://go-review.googlesource.com/c/go/+/737620
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Mark Freeman <markfreeman@google.com>