[Sema] Catch use-before-declarations in nested closures
Previously we would allow these in Sema and diagnose them in SILGen,
but allowing them in Sema is unsound because it means the constraint
system ends up kicking interface type requests for declarations that
should be type-checked as part of the closure itself. Adjust the
name lookup logic to look through parent closures when detecting
invalid forward references.
For now we don't fallback to an outer result on encountering a
use-before-declaration to preserve the current behavior. I'm planning
on changing that in the next commit though.
rdar://74430478
