cmd/compile: cleanup Func construction

cmd/compile: cleanup Func construction

This CL moves more common Func-setup logic into ir.NewFunc. In
particular, it now handles constructing the Name and wiring them
together, setting the Typecheck bit, and setting Sym.Func.

Relatedly, this CL also extends typecheck.DeclFunc to append the
function to typecheck.Target.Funcs, so that callers no longer need to
do this.

Change-Id: Ifa0aded8df0517188eb295d0dccc107af85f1e8a
Reviewed-on: https://go-review.googlesource.com/c/go/+/520338
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Auto-Submit: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>