[CodeCompletion] Move implementation of solver-based completions to their own functions

This has the benefit that in case of a code completion crash, we can see what kind of completion crashed without having to symbolicate the stack trace using LLDB.

It also made `trySolverCompletion` small enough that we can inline it into `doneParsing`, which makes debugging easier because you won’t accidentally step over the call to `trySolverCompletion`, which actually performs the completion.

Author: ahoppen