go/types: permit type elision from composite literal map keys

Per pending https://go-review.googlesource.com/2591 .
Depends on: https://go-review.googlesource.com/2621 .

Change-Id: I2aa82916454504b5852518b3ef7fc8637126d0c7
Reviewed-on: https://go-review.googlesource.com/2623
Reviewed-by: Alan Donovan <adonovan@google.com>
2 files changed