[Bash-autocomplete] Refactor autocomplete code into own function

Summary:
We wrote many codes in HandleImediateArgs, so I've refactored it into
handleAutocompletions.

Reviewers: v.g.vassilev, teemperor

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D37249

llvm-svn: 312018