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

[Vassil Vassilev via cfe-commits]

vgvassilev wrote:

> > Can we test how this deduplication logic behaves with header units? I believe the share the same code path but just to make sure this case is covered for standard c++ modules.
> 
> I can test this and let you know. Do you think it should be added as one of the tests in this PR? Or it would be repetitive since the clang modules one is already in

A test or two wouldn’t hurt. 

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