cmd/compile: fixes for non-constant Sizeof/Alignof/Offsetof

Includes Robert's suggested fix in validate.go to not fail on
non-constant alignof/offsetof/sizeof calls. Further changes to wait on
transforming these calls until stenciling time, when we can call
EvalConst() to evaluate them once all the relevant types are known.

Added a bunch of new tests for non-constant Sizeof/Alignof/Offsetof.

Fixes #47716

Change-Id: I469af888eb9ce3a853124d919eda753971009b3e
Reviewed-on: https://go-review.googlesource.com/c/go/+/344250


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