[clang] [CIR][NFC] Use arrangeFunctionDeclaration to build function types (PR #139787)

[Erich Keane via cfe-commits]

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

WithExtraSlots is still kinda 'meh' for a name, but at least a bit better.  WOuld love a better comment on them to just explain what is going on (like what non-formal arguments means/), but good enough for now.

https://github.com/llvm/llvm-project/pull/139787