[Serialization] Introduce OnDiskHashTable for specializations

Following up for https://github.com/llvm/llvm-project/pull/83108

This follows the suggestion literally from
https://github.com/llvm/llvm-project/pull/76774#issuecomment-1951172457

which introduces OnDiskHashTable for specializations based on D41416.

Note that I didn't polish this patch to reduce the diff from D41416 to
it easier to review. I'll make the polishing patch later. So that we can
focus what we're doing in this patch and focus on the style in the next
patch.