c++/modules: Catch exposures of TU-local values through inline references [PR119996]

In r15-9136-g0210bedf481a9f we started erroring for inline variables
that exposed TU-local entities in their definition, as such variables
would need to have their definitions emitted in importers but would not
know about the TU-local entities they referenced.

A case we mised was potentially-constant references, which disable
streaming of their definitions in make_dependency so as to comply with
[expr.const] p9.2.  This meant that we didn't see the definition
referencing a TU-local entity, leading to nonsensical results.

	PR c++/119551
	PR c++/119996

gcc/cp/ChangeLog:

	* module.cc (depset::hash::make_dependency): Also mark inline
	variables referencing TU-local values as exposures here.
	(depset::hash::finalize_dependencies): Add error message for
	inline variables.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/internal-13.C: New test.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
Reviewed-by: Jason Merrill <jason@redhat.com>
(cherry picked from commit 22ccaded63e96e5a42f4e3676dbbb57aa05b36f9)