[clang] [Clang][ItaniumMangle][NFC] Refactor FunctionTypeDepthState (PR #196240)

[Oliver Hunt via cfe-commits]

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

This looks like a good improvement to me - it took me a while to go through the cursed bit manipulation, but the logic all seems correct to me.

I don't know about the renaming - I'm not sufficiently familiar with mangling terminology so I'd like @erichkeane or @shafik to ok that separately

https://github.com/llvm/llvm-project/pull/196240