save LTO import information and check it when trying to reuse build products.

adopts simple strategy devised with assistance from mw: Instead of accumulating
(and acting upon) LTO import information over an unbounded number of prior
compilations, just see if the current import set matches the previous import set.
if they don't match, then you cannot reuse the PostLTO build product for that
module.

In either case (of a match or a non-match), we can (and must) unconditionally
emit the current import set as the recorded information in the incremental
compilation cache, ready to be loaded during the next compiler run for use in
the same check described above.

resolves issue 59535.

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