[lldb/Commands] Add ability to run shell command on the host.

This patch introduces the `(-h|--host)` option to the `platform shell`
command. It allows the user to run shell commands from the host platform
(always available) without putting lldb in the background.

Since the default behaviour of `platform shell` is to run the command of
the selected platform, having such a choice can be quite handy when
debugging remote targets, for instances.

This patch also introduces a `shell` alias, to improve the command
discoverability and make it more convenient to use for the user.

rdar://62856024

Differential Revision: https://reviews.llvm.org/D79659



Signed-off-by: Med Ismail Bennani <medismail.bennani@gmail.com>