c++: Fix MODULE_VERSION breakage

c++: Fix MODULE_VERSION breakage

Adding includes to module.cc triggered the kind of build failure I
wanted to check for.  In this case it was MODULE_VERSION not being
defined, and module.cc's internal #error triggering.  I've relaxed the
check in Make-lang, so we proviude MODULE_VERSION when DEVPHASE is not
empty (rather than when it is 'experimental').  AFAICT devphase is
empty for release builds, and the #error will force us to decide
whether modules is sufficiently baked at that point.

	gcc/cp
	* Make-lang.in (MODULE_VERSION): Override when DEVPHASE not empty.
	* module.cc: Comment.