[Bash-autocompletion] Add support for -std=

Summary:
Add support for autocompleting values of -std= by including
LangStandards.def. This patch relies on D36782, and is using two-stage
code generation.

Reviewers: v.g.vassilev, teemperor, ruiu

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D36820