This PR is a follow-up to #128655.
It adds another test to ensure deterministic ordering in `.mod` files
and includes related changes to prevent non-deterministic ordering
caused by iterating over a set ordered by heap pointers. This issue is
particularly noticeable when using Flang as a library and compiling the
same files multiple times.
The reduced test case is as minimal as possible. We were unable to
reproduce the issue with a smaller set of files.
6eefadd8ef