cmd/compile: fix nongeneric closures in generic functions

Ensure that formal parameter Names are correctly copied and marked
with the correct Curfn. We need to ensure this even when the underlying
closure has no type parameters.

(Aside: it is strange that the types of things contain formal
parameter names that need to be copied. Maybe that's an underlying
larger problem that needs to be fixed.)

Fixes #45738

Change-Id: Ia13d69eea992ff7080bd44065115bc52eb624e73
Reviewed-on: https://go-review.googlesource.com/c/go/+/313652


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