cmd/compile: workaround inlining of closures with type switches

Within clovar, n.Defn can also be *ir.TypeSwitchGuard. The proper fix
here would be to populate m.Defn and have it filled in too, but we
already leave it nil in inlvar. So for consistency, this CL does the
same in clovar too.

Eventually inl.go should be rewritten to fully respect IR invariants.

Fixes #45743.

Change-Id: I8b38e5d8b2329ad242de97670f2141f713954d28
Reviewed-on: https://go-review.googlesource.com/c/go/+/313289


Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dan Scales <danscales@google.com>
Trust: Dan Scales <danscales@google.com>
Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>