go/types: avoid repeated "declared but not used" errors for closure variables

go/types: avoid repeated "declared but not used" errors for closure variables

At the end of type-checking a function or closure, unused local variables
are reported by looking at all variables in the function scope and its
nested children scopes. If a nested scope belonged to a nested function
(closure), that scope would be searched twice, leading to multiple error
messages for unused variables.

This CL introduces an internal-only marker to identify function scopes
so that they can be ignored where needed.

Fixes #22524.

Change-Id: If58cc17b2f0615a16f33ea262f50dffd0e86d0f0
Reviewed-on: https://go-review.googlesource.com/75251
Reviewed-by: Alan Donovan <adonovan@google.com>