[clang] [llvm] [clang][Modules][Serialization] Avoid serializing duplicate source location entries for shared input files (PR #209795)

[Corentin Jabot via cfe-commits]

cor3ntin wrote:

> Just added them to the top of the PR description.

Can we somehow get regression tests for these?

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