cmd/compile: add -lang flag to specify language version

cmd/compile: add -lang flag to specify language version

The default language version is the current one.

For testing purposes, added a check that type aliases require version
go1.9. There is no consistent support for changes made before 1.12.

Updates #28221

Change-Id: Ia1ef63fff911d5fd29ef79d5fa4e20cfd945feb7
Reviewed-on: https://go-review.googlesource.com/c/144340
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>