c++: Modularise start_cleanup_fn [PR98893]

'start_cleanup_fn' is not currently viable in modules, due to generating
functions relying on the 'start_cleanup_cnt' counter which is reset to 0
with each new TU.  This means that cleanup functions declared in a TU
will conflict with any imported cleanup functions.

This patch mitigates the problem by using the mangled name of the decl
we're destroying as part of the name of the function.  This should avoid
clashes unless the decls would have clashed anyway.

	PR c++/98893

gcc/cp/ChangeLog:

	* decl.cc (start_cleanup_fn): Make name from the mangled name of
	the passed-in decl.
	(register_dtor_fn): Pass decl to start_cleanup_fn.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/pr98893_a.H: New test.
	* g++.dg/modules/pr98893_b.C: New test.

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