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

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

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

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



Co-authored-by: Eric Schweitz <eschweitz@nvidia.com>

Differential Revision: https://reviews.llvm.org/D113023