R=go1.11

Functions (at the package level) were collected and their bodies
type-checked after all other package-level objects were checked.
But function literals where type-checked right away when they were
encountered so that they could see the correct, partially populated
surrounding scope, and also to mark variables of the surrounding
function as used.

This approach, while simple, breaks down in esoteric cases where
a function literal appears inside the declaration of an object
that its body depends on: If the body is type-checked before the
object is completely set up, the literal may use incomplete data
structures, possibly leading to spurious errors.

This change postpones type-checking of function literals to later;
after the current expression or statement, but before any changes
to the enclosing scope (so that the delayed type-checking sees the
correct scope contents).

The new mechanism is general and now is also used for other
(non-function) delayed checks.

Fixes #22992.

Change-Id: Ic95f709560858b4bdf8c645be70abe4449f6184d
Reviewed-on: https://go-review.googlesource.com/83397Reviewed-by: Alan Donovan <adonovan@google.com>