[flang] Rework host runtime folding and enable REAL(2) folding with it.

- Rework the host runtime table so that it is constexpr to avoid
  having to construct it and to store/propagate it.
- Make the interface simpler (remove many templates and a file)
- Enable 16bits float folding using 32bits float host runtime
- Move StaticMultimapView into its own header to use it for host
  folding

Reviewed By: klausler, PeteSteinfeld

Differential Revision: https://reviews.llvm.org/D88981