[dev.typeparams] cmd/compile: set Func.ClosureCalled in escape analysis

The Func.ClosureCalled flag is an optimization used by escape analysis
to detect closures that were directly called, so we know we have
visibility of the result flows. It's not needed by any other phases of
the compiler, so we might as well calculate it within escape analysis
too.

This saves some trouble during IR construction and trying to maintain
the ClosureCalled flag through inlining and copying.

Passes toolstash -cmp.

Change-Id: Ic53cecb7ac439745c0dfba2cd202b9cc40f1e47c
Reviewed-on: https://go-review.googlesource.com/c/go/+/332691


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