c++: Avoid UB in signed shift [PR 98625]

[Nathan Sidwell]

Thanks Jakub & Martin for unconfusing me about where the UB was!

I'd forgotten that left	shifting a negative value is UB	until C++20.
Insert some casts to do	unsigned shifts.

	PR c++/98625
	gcc/cp/
         * module.cc (bytes_in::i, bytes_in::wi): Avoid left shift of
         signed type.

-- 
Nathan Sidwell
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