If an LLVM module's exports change, cannot reuse its post-LTO object file in

If an LLVM module's exports change, cannot reuse its post-LTO object file in
incremental compilation.

This is symmetric to PR #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 #69798