cmd/compile: set types properly for imported funcs with closures

For the new export/import of node types, we were just missing setting
the types of the closure variables (which have the same types as the
captured variables) and the OCLOSURE node itself (which has the same
type as the Func node).

Re-enabled inlining of functions with closures.

Change-Id: I687149b061f3ffeec3244ff02dc6e946659077a9
Reviewed-on: https://go-review.googlesource.com/c/go/+/308974


Trust: Dan Scales <danscales@google.com>
Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>