[ThinLTO]Sort imported GUIDs before cache key update (#92622)

Add 'sort' here since it's helpful when container type
changes (for example, https://github.com/llvm/llvm-project/pull/88024
wants to change container type from `unordered_set` to `DenseMap)

@MaskRay points out `std::` doesn't randomize the iteration order of
`unordered_{set,map}`, and the iteration order for single build is
deterministic.