[dev.typeparams] cmd/compile: move //go:embed -lang check to noder

User errors should be reported in noder and/or typecheck, we already
know the -lang flag's value during noding, and checking it then works
better for unified IR.

The "multiple files for type" and "cannot apply to var of type" errors
should also be moved to typecheck, but then they'd have to be
duplicated for -G=3 mode (because it avoids typecheck). So those are
left behind for now.

Change-Id: I7caf16163c9faf975784acacdb8147514d2e698e
Reviewed-on: https://go-review.googlesource.com/c/go/+/327609


Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>