[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
git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@309113
91177308-0d34-0410-b5e6-
96231b3b80d8