[clang] [llvm] [clang][Modules][serialization] Deduplicate source location entries across loaded modules (PR #209795)

[Jonas Hahnfeld via llvm-commits]

hahnjo wrote:

> @hahnjo, this might be also relevant for ROOT and worth testing.

I gave this a quick try and it's failing in the build, with an assertion:
```
clang/include/clang/Basic/SourceLocation.h:330: const char *clang::PresumedLoc::getFilename() const: Assertion `isValid()' failed.
```
This happens here: https://github.com/root-project/root/blob/dfe7f8018320b93fab670705226a678442b6a901/core/dictgen/src/BaseSelectionRule.cxx#L41-L59
AFAICT we've never seen (so far) the case where a `SourceLocation` was valid, but then the `PresumedLoc` is invalid...

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