[clang] [flang] [llvm] [AArch64][Driver] Better handling of target feature dependencies (PR #78270)

[Sjoerd Meijer via cfe-commits]

https://github.com/sjoerdmeijer approved this pull request.

This is a very big patch, but it makes a lot of sense. The idea to do it in this way is clearly an improvement. There are quite a number of (new) moving parts involved here, but look reasonable to me so let's give this is a try.  

Thanks for cleaning up the mess (also my mess). 

https://github.com/llvm/llvm-project/pull/78270