[clang] [llvm] [clang][Modules][serialization] Deduplicate source location entries across loaded modules (PR #209795)

[Jonas Hahnfeld via cfe-commits]

hahnjo wrote:

> Hi @hahnjo, you were right, I was building it wrong. Apologies!
> 
> Did some debugging and found the cause of the crash. The patch makes the local→global source-location map piecewise, but the inverse map used when re-serializing a loaded location was still a flat subtraction. This caused locations to do a a read-write-read round trip with an incorrect offset and eventually resolve into the wrong file.
> 
> The fix records the exact per-file amount added on the way in so the inverse subtracts the same thing.
> 
> It's now on the branch. Could you please try building again?

Yes, this seems to finish building. However, it seems to slow down startup of the ROOT interpreter and also increase max RSS by around 15 MB - is that expected?

https://github.com/llvm/llvm-project/pull/209795