[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
diff --git a/clang/lib/Driver/Driver.cpp b/clang/lib/Driver/Driver.cpp
index b8cb16f..5eaa483 100644
--- a/clang/lib/Driver/Driver.cpp
+++ b/clang/lib/Driver/Driver.cpp
@@ -1297,7 +1297,7 @@
     std::sort(SuggestedCompletions.begin(), SuggestedCompletions.end(),
               [](StringRef A, StringRef B) { return A.compare_lower(B) < 0; });
 
-    llvm::outs() << llvm::join(SuggestedCompletions, " ") << '\n';
+    llvm::outs() << llvm::join(SuggestedCompletions, "\n") << '\n';
     return false;
   }