…file in

incremental compilation.

This is symmetric to PR rust-lang#67020, which handled the case where the LLVM module's
*imports* changed. This commit builds upon the infrastructure added there; the
export map is just the inverse of the import map, so we can build the export map
at the same time that we load the serialized import map.

Fix rust-lang#69798

Namely, a regression test for issue rust-lang#69798 (export added), and the inverse of
that test (export removd).

Renamed the struct to make it a little clearer that it doesn't just hold one
imports map. (I couldn't bring myself to write it as `ThinLTOImportsExports`
though, mainly since the exports map is literally derived from the imports map
data.) Added some doc to the struct too.

Revised comments to add link to the newer issue that discusses why the exports
are relevant.

Renamed a few of the methods so that the two character difference is more
apparent (because 1. the method name is shorter and, perhaps more importantly,
the changed characters now lie at the beginning of the method name.)