Fix and simplify lldb.command decorator

Summary:
This change fixes one issue with `lldb.command`, and also reduces the implementation.

The fix: a command function's docstring was not shown when running `help <command_name>`. This is because the docstring attached the source function is not propagated to the decorated function (`f.__call__`). By returning the original function, the docstring will be properly displayed by `help`.

Also with this change, the command name is assumed to be the function's name, but can still be explicitly defined as previously.

Additionally, the implementation was updated to:

* Remove inner class
* Remove use of `inspect` module
* Remove `*args` and `**kwargs`

Reviewers: clayborg

Reviewed By: clayborg

Subscribers: keith, xiaobai, lldb-commits

Differential Revision: https://reviews.llvm.org/D48658

llvm-svn: 336287