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

This PR adds a number of cases to the switch statement in
`CIRGenBUiltin.cpp`. Some existing cases were relocated, so the order
matches the order from the switch statement in clangs codegen.
Additionally, some exisiting cases were moved to functions, to keep the
code a little cleaner. In the future, it will be easier to keep track of
which builtins have not been implemented, since there would always be a
NYI case for unimplemented builtins.

Author: HendrikHuebner