c++/modules: Remember that header units have CMIs

c++/modules: Remember that header units have CMIs

This appears to be an oversight in the definition of module_has_cmi_p.
This change will allow us to use the function directly in more places
that need to additional work only if generating a module CMI in the
future, allowing us to do additional work only when we know we need it.

gcc/cp/ChangeLog:

	* cp-tree.h (module_has_cmi_p): Also include header units.
	(module_maybe_has_cmi_p): Update comment.
	* module.cc (set_defining_module): Only need to track
	declarations for later exporting if the module may have a CMI.
	(set_defining_module_for_partial_spec): Likewise.
	* name-lookup.cc (pushdecl): Likewise.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>