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

[Haojian Wu via cfe-commits]

hokein wrote:

Thanks for implementing this!

The 40% saving on main looks very promising. As this is a substantial change, we should consider more tests before landing it. @alexfh might be able to help validate and test this within Google internal.

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