Fixed false ThinLTO cache misses problem (PR 45819).

We relied on the fact that the iterators walks through the elements of a
DenseSet in a deterministic order (which is not true). This caused
ThinLTO cache misses. This patch addresses this problem.
See PR 45819 for additional information
https://bugs.llvm.org/show_bug.cgi?id=45819

Differential Revision: https://reviews.llvm.org/D79772