[clang] [llvm] Update llvm::Registry to work for LLVM shared library builds on windows (PR #109024)

[Thomas Fransham via cfe-commits]

fsfod wrote:

> These changes LGTM but precommit CI was unable to run, so it would be nice if that could be re-run to ensure it comes back green before landing.

It won't be able to pass until https://github.com/llvm/llvm-project/pull/108276 is merged.

https://github.com/llvm/llvm-project/pull/109024