c++: Fix indirect partitions [PR 98944]

[Nathan Sidwell]

The most recent reimplementation of module loading initialization
changed the behaviour of setting an	import's location, and broke some
partition handling.

         PR c++/98944
         gcc/cp/
         * module.cc (module_state::is_rooted): Rename to ...
         (module_state::has_location): ... here.	 Adjust	callers.
	(module_state::read_partitions): Adjust	validity check.
         Don't overwrite	a known	location.
         gcc/testsuite/
         * g++.dg/modules/pr98944_a.C: New.
         * g++.dg/modules/pr98944_b.C: New.
         * g++.dg/modules/pr98944_c.C: New.
         * g++.dg/modules/pr98944_d.C: New.


-- 
Nathan Sidwell
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