[lldb] Add description to option completions.

Summary:
Right now our argument completions are rather cryptic for command options as they only list the letters:

```
(lldb) breakpoint set -
Available completions:
	-G
	-C
	-c
	-d
	-i
	-o
	-q
	-t
	-x
[...]
```

With the new completion API we can easily extend this with the flag description so that it looks like this now:

```
(lldb) breakpoint set -
Available completions:
	-G -- The breakpoint will auto-continue after running its commands.
	-C -- A command to run when the breakpoint is hit, can be provided more than once, the commands will get run in order left to right.
	-c -- The breakpoint stops only if this condition expression evaluates to true.
	-d -- Disable the breakpoint.
	-i -- Set the number of times this breakpoint is skipped before stopping.
	-o -- The breakpoint is deleted the first time it stop causes a stop.
	-q -- The breakpoint stops only for threads in the queue whose name is given by this argument.
	-t -- The breakpoint stops only for the thread whose TID matches this argument.
	-x -- The breakpoint stops only for the thread whose index matches this argument.
```

The same happens with --long-options now.

Reviewers: #lldb, labath

Reviewed By: labath

Subscribers: labath, JDevlieghere, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D67063

llvm-svn: 370628