[clang] [CIR] floating-point, pointer, and function types (PR #120484)

[Ronan Keryell via cfe-commits]

keryell wrote:

> My bad here, we usually don't accept such changes and I missed that during review time. ClangIR policy to changes outside CIR is to upstream them to MLIR and after that's done we cherry-pick it to the incubator (so rebases are smooth). I'll have to revert your change, sorry for the churn!

@bcardosolopes I can work on the phase 2 of the change I had in mind and remove also the `!void` from function type textual IR to avoid adding a new feature in the MLIR parsing infrastructure.

https://github.com/llvm/llvm-project/pull/120484