[clang] [WIP] Implement `print-deserialized-declarations` flag to dump source… (PR #133910)

[Ilya Biryukov via cfe-commits]

https://github.com/ilya-biryukov requested changes to this pull request.

Thanks, this looks like a good start! I have left a few comments and also feel there are a few things missing:
- basic tests
- the code registering deserialization listener
- suggestion: a commit message could benefit from a description of how we are going to use this flag (heuristics for minimization tools), describe the output format and mention that the output stability is not guaranteed (it's also another reason why to put it behind a `-cc1` flag) 

https://github.com/llvm/llvm-project/pull/133910