Instead of trying to validate map key types eagerly in some
cases, delay their validation to the end of type-checking,
when we all type information is present.

Passes go build -toolexec 'toolstash -cmp' -a std .

Fixes #21273.
Fixes #21657.

Change-Id: I532369dc91c6adca1502d6aa456bb06b57e6c7ff
Reviewed-on: https://go-review.googlesource.com/75310Reviewed-by: Matthew Dempsky <mdempsky@google.com>