[flang][driver] Add support for generating LLVM bytecode files

Support for generating LLVM BC files is added in Flang's compiler and
frontend drivers. This requires the `BitcodeWriterPass` pass to be run
on the input LLVM IR module and is implemented as a dedicated frontend
aciton. The new functionality as seen by the user (compiler driver):
```
flang-new -c -emit-llvm file.90
```
or (frontend driver):
```
flang-new -fc1 -emit-llvm-bc file.f90
```

The new behaviour is consistent with `clang` and `clang -cc1`.

Differential Revision: https://reviews.llvm.org/D123211