c++/modules: Make bitfield storage unit detection more robust

c++/modules: Make bitfield storage unit detection more robust

Modules streaming needs to handle these differently from other unnamed
FIELD_DECLs that are streamed for internal RECORD_DECLs, and there
doesn't seem to be a good way to detect this case otherwise.

This matters only to allow for compiler-generated type definitions that
build FIELD_DECLs with no name, as otherwise they get confused.
Currently the only such types left I hadn't earlier fixed by giving
names to are contextless, for which we have an early check to mark their
fields as MK_unique anyway, but there may be other cases in the future.

gcc/cp/ChangeLog:

	* module.cc (trees_out::walking_bit_field_unit): New flag.
	(trees_out::trees_out): Initialize it.
	(trees_out::core_vals): Set it.
	(trees_out::get_merge_kind): Use it, move previous ad-hoc check
	into assertion.

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