[lldb/Plugins] Fix method dispatch bug when using multiple scripted processes

This patch should address a bug when a user have multiple scripted
processes in the same debugging session.

In order for the scripted process plugin to be able to call into the
scripted object instance methods to fetch the necessary data to
reconstruct its state, the scripted process plugin calls into a
scripted process interface, that has a reference to the created script
object instance.

However, prior to this patch, we only had a single instance of the
scripted process interface, living the script interpreter. So every time
a new scripted process plugin was created, it would overwrite the script
object instance that was held by the single scripted process interface
in the script interpreter.

That would cause all the method calls made to the scripted process
interface to be dispatched by the last instanciated script object
instance, which is wrong.

In order to prevent that, this patch moves the scripted process
interface reference to be help by the scripted process plugin itself.

rdar://104882562

Differential Revision: https://reviews.llvm.org/D143308



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