[clang] First run at removing the linked-list for decls. Tests not run, but … (PR #113983)

[Erich Keane via cfe-commits]

erichkeane wrote:

Clang format fails, I expected this, there is a ton of TODOs, commented out code, etc anyway, so I'll fix that after this 'proof of concept' shows promise.

https://github.com/llvm/llvm-project/pull/113983