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

[Qiongsi Wu via cfe-commits]

qiongsiwu wrote:

I ran `4b9d839d` locally last week and I only saw about 1% regression in scan time. I will test the latest branch over the next day or so. 

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