[dev.typeparams] cmd/compile: start adding info needed for typeparams in types & ir

[dev.typeparams] cmd/compile:  start adding info needed for typeparams in types & ir

We are focusing on generic functions first, and ignoring type lists for
now.

The signatures of types.NewSignature() and ir.NewCallExpr() changed (with
addition of type args/params).

Change-Id: I57480be3d1f65690b2946e15dd74929bf42873f2
Reviewed-on: https://go-review.googlesource.com/c/go/+/287416
Reviewed-by: Robert Griesemer <gri@golang.org>
Trust: Robert Griesemer <gri@golang.org>
Trust: Dan Scales <danscales@google.com>