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

[Qiongsi Wu via cfe-commits]

qiongsiwu wrote:

> @qiongsiwu Have you tested scanning performance? I skimmed over the PR and saw that `buildLoadedInputFiles()` gets called during AST writing, which might deserialize more than before.

Yeah good point. I tested a TU with 212 imports from the macOS SDK. Scanning regressed about 4.74% (1.917s - 2.009s). Building the explicit modules is also a bit slower (2%, 11.10s to 11.34s). 

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