[clang] [clang] Propagate definition data to all redecls (PR #170090)

[Jonas Hahnfeld via cfe-commits]

hahnjo wrote:

> Do you have a good explanation of why the assertion failure started manifesting only after this commit?

Yes, before this change we were only going over all previous declarations in a single direction. By using the iterator returned from `redecls()`, we wrap around the chain until we visited all decls. However, it's an error to see the first decl twice, which can happen if the decl chain is invalidated during iteration.

https://github.com/llvm/llvm-project/pull/170090