[dev.typeparams] cmd/compile: refactor noder/irgen helpers

This CL refactors the code for invoking the types2 checker and for
validating //go:embed directives to be easier to reuse separately.
No functional change.

Change-Id: I706f4ea4a26b1f1d2f4064befcc0777a1067383d
Reviewed-on: https://go-review.googlesource.com/c/go/+/323310


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