[clang] [analyzer][NFCI] Move the core.FixedAddressDereference checker to optin.core (PR #181858)

[Donát Nagy via cfe-commits]

NagyDonat wrote:

> I'm just moving it around; this is also why I'd push back accepting changes for it now. This is not the right time.

Completely fair, I agree that creating a separate PR is a more elegant place for those changes.

https://github.com/llvm/llvm-project/pull/181858