cmd/compile: don't re-typecheck while importing

The imported code is already typechecked. NodAddrAt typechecks its
argument, which is unnecessary here and leads to errors when
typechecking unexported field references in other packages' code.

Mark the node is question as already typechecked, so we don't
retypecheck it.

Fixes #50148

Change-Id: I9789e3e7dd4d58ec095675e27b1c98389f7a0c44
Reviewed-on: https://go-review.googlesource.com/c/go/+/371554


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