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

[Vlad Serebrennikov via cfe-commits]

Endilll wrote:

> Does this affect diagnostic output? I thought we had to use separate slocs so we could distinguish which module the textual header came from. Is it more like, in the graph, A, B, C, where B -> A, C -> A, and all nodes have foo.h, we use the slocs for foo.h from A? i.e. we find the first sloc in the module where it was introduced?

I know the author was aware of this aspect (see [this review](https://github.com/ayokunle321/llvm-project/pull/1#pullrequestreview-4628570060)), and I'm also wondering that the approach is. I think it would be worth covering in the PR description.

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