[flang][driver] Add support for `-emit-llvm`

This patch adds support for the `-emit-llvm` option in the frontend
driver (i.e. `flang-new -fc1`). Similarly to Clang, `flang-new -fc1
-emit-llvm file.f` will generate a textual LLVM IR file.

Depends on D118985

Differential Revision: https://reviews.llvm.org/D119012