c++: duplicate const static members [PR 99283]

c++: duplicate const static members [PR 99283]

This is the bug that keeps on giving.  Reducing it has been successful
at hitting other defects. In this case, some more specialization hash
table fun, plus an issue with reading in a definition of a duplicated
declaration.  At least I discovered a null context check is no longer
needed.

	PR c++/99283
	gcc/cp/
	* module.cc (dumper::operator): Make less brittle.
	(trees_out::core_bools): VAR_DECLs always have a context.
	(trees_out::key_mergeable): Use same_type_p for asserting.
	(trees_in::read_var_def): Propagate
	DECL_INITIALIZED_BY_CONSTANT_EXPRESSION_P.
	gcc/testsuite/
	* g++.dg/modules/pr99283-5.h: New.
	* g++.dg/modules/pr99283-5_a.H: New.
	* g++.dg/modules/pr99283-5_b.H: New.
	* g++.dg/modules/pr99283-5_c.C: New.