The variables on the lhs of a short variable declaration are
only in scope after the variable declaration. Specifically,
function literals on the rhs of a short variable declaration
must not see newly declared variables on the lhs.

This used to work and this bug was likely introduced with
https://go-review.googlesource.com/c/go/+/83397 for go1.11.
Luckily this is just an oversight and the fix is trivial:
Simply use the mechanism for delayed type-checkin of function
literals introduced in the before-mentioned change here as well.

Fixes #24026.

Change-Id: I74ce3a0d05c5a2a42ce4b27601645964f906e82d
Reviewed-on: https://go-review.googlesource.com/96177Reviewed-by: Alan Donovan <adonovan@google.com>