[dev.typeparams] cmd/compile: simplify NewClosureFunc

I initially made NewClosureFunc take an "outerfn *Func" parameter
because I was planning on having it handle closure naming, until
remembering that naming needs to wait until typecheck for noder.

We don't actually need the *Func yet, just to know whether it's
non-nil. So change the parameter to a bool, which simplifies callers a
little.

Change-Id: Ie83ee4a1ed0571ac6d3879ffd8474c6c3c1a9ff9
Reviewed-on: https://go-review.googlesource.com/c/go/+/327450


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