[ScriptInterpreter] Pass the debugger instead of the command interpreter

As discussed in D61090, there's no good reason for the script
interpreter to depend on the command interpreter. When looking at the
code, it becomes clear that we mostly use the command interpreter as a
way to access the debugger. Hence, it makes more sense to just pass that
to the script interpreter directly.

This is part 1 out of 2. I have another patch in the pipeline that
changes the ownership of the script interpreter to the debugger as well,
but I didn't get around to finish that today.

Differential revision: https://reviews.llvm.org/D61172

llvm-svn: 359330