[dev.typeparams] cmd/compile: add and use ir.RawOrigExpr

This CL adds ir.RawOrigExpr, which can be used to represent arbitrary
constant expressions without needing to build and carry around an
entire IR representation of the original expression. It also allows
distinguishing how the constant was originally written by the
user (e.g., "0xff" vs "255").

This CL then also updates irgen to make use of this functionality for
expressions that were constant folded by types2.

Change-Id: I41e04e228e715ae2735c357b75633a2d08ee7021
Reviewed-on: https://go-review.googlesource.com/c/go/+/323210


Trust: Matthew Dempsky <mdempsky@google.com>
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
Reviewed-by: Dan Scales <danscales@google.com>