[llvm] [llvm] Reduce memory footprint of Debug metadata nodes (PR #71227)

[Adrian Prantl via llvm-commits]

adrian-prantl wrote:

Let me rephrase that — could you document how SubclassData is used by the nodes you're changing in this patch? For example in the top-level doxygen comment of each DI* class you are modifying?

https://github.com/llvm/llvm-project/pull/71227