[flang][CodeGen] Transform `fir.call` to `llvm.call`

This patch extends the `FIRToLLVMLowering` pass in Flang by adding a
hook to transform `fir.call` to `llvm.call`.

This is part of the upstreaming effort from the `fir-dev` branch in [1].

[1] https://github.com/flang-compiler/f18-llvm-project



Patch originally written by:
Co-authored-by: Eric Schweitz <eschweitz@nvidia.com>
Co-authored-by: V Donaldson <vdonaldson@nvidia.com>

Differential Revision: https://reviews.llvm.org/D113278