[Bash-autocompletion] Show HelpText with possible flags

Summary:
`clang --autocomplete=-std` will show
```
-std:   Language standard to compile for
-std=   Language standard to compile for
-stdlib=        C++ standard library to use
```
after this change.

However, showing HelpText with completion in bash seems super tricky, so
this feature will be used in other shells (fish, zsh...).

Reviewers: v.g.vassilev, teemperor, ruiu

Subscribers: cfe-commits, hiraditya

Differential Revision: https://reviews.llvm.org/D35759

llvm-svn: 309113