[clang] [clang][AST] Fix generation with multiple external sources (PR #219189)

[Vassil Vassilev via cfe-commits]

vgvassilev wrote:

> > > In my opinion no, it's just 60 lines of whitebox test code for a 10-line function, with an external source that is not even part of a chain of external sources. It doesn't show the actual issue of what happens with the current bug.
> > 
> > 
> > I agree it does not reproduce the problem in ROOT, however, this seems to expose the problem which is good enough for me to move forward with this change.
> 
> OK, I'll add a slightly adapted test based on what you posted. Can you let me know if you used generative AI to produce it, so I can label it according to the project policies?

Yes, that's generated and checked by claude.

https://github.com/llvm/llvm-project/pull/219189