Support decompilation of LLVM bitcode files within `retdec-decompiler`

I mean in a simplified fashion like in `llvm2c`, where one only has to provide a bitcode file and path to an output file (but I guess it can be inferred).

Start by reading the retdec-decompiler entry point and comparing its inputs with the simplified llvm2c workflow described in the issue. Determine how an LLVM bitcode file and output path should be accepted, then verify that decompilation produces the requested output; the issue names no tests or implementation files.

Written by the indexing model from the issue text.