[clang] [CIR] Add NYI cases to builtin switch statement and move existing cases into functions (PR #168699)

[Andy Kaylor via cfe-commits]

andykaylor wrote:

I just merged https://github.com/llvm/llvm-project/pull/166037 which, unfortunately is going to require a bit of rebase work to fit with this change. On the plus side, it implements the correct builtin handling for `__builtin_isnan`.

https://github.com/llvm/llvm-project/pull/168699