[dev.typeparams] cmd/compile: make sure closures inside generic funcs are not compiled

Closures inside generic functions were being added to the g.target.Decls
list during noding, just like other closures. We remove generic
functions/methods from g.target.Decls, so they don't get compiled
(they're only available for export and stenciling). Most closures inside
generic functions/methods were similarly being removed from
g.target.Decls, because they have a generic parameter. But we need to
ensure no closures in generic function/methods are left remaining in
g.target.Decls, since we don't want them transformed and compiled.

So, we set a flag in (*irgen) that records when we are noding a
top-level generic function/method, and don't add any closures to
g.target.Decls when the flag is true.

Updates #47514

Change-Id: Id66b4c41d307ffa8f54cab6ce3646ade81606862
Reviewed-on: https://go-review.googlesource.com/c/go/+/340258


Trust: Dan Scales <danscales@google.com>
Reviewed-by: Keith Randall <khr@golang.org>