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

[Vassil Vassilev via cfe-commits]

vgvassilev wrote:

> Tested in our internal workload and the result looks not bad (no crash). This is not a LGTM and I want to take a close look for this. Thanks for your patience in ahead.

Can you provide maybe some performance numbers, too?

@ayokunle321, the other thing I was thinking about is that now when we can detect duplicates maybe we can issue better diagnostics advising users how to avoid such duplications. 

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