[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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@312018 91177308-0d34-0410-b5e6-96231b3b80d8