[clang] [clang][tools] Handle weak (and duplicate) symbols in clang-extdef-mapping (PR #222010)

[Donát Nagy via cfe-commits]

https://github.com/NagyDonat commented:

The change looks good and I think this is a good approach to handle this problem with the weak symbols in CTU analysis. However, I found simplification opportunities (see inline comments) and I'm not familiar enough with `clang-extdef-mapping` to give a formal approval. Please try to find a reviewer who is familiar with this part of the codebase.

https://github.com/llvm/llvm-project/pull/222010