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

[Balázs Benics via cfe-commits]

steakhal wrote:

I realised I might need to elaborate about what has changed here.
 - Just moved stuff around - as-is. Didn't change anything.
 - In the tests I decided to clean up some useless `alpha.core` enablements, or be explicit and selective what you enable from that package.
 
 Otherwise it does what you think it does.

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