[lldb/Plugins] Add ScriptedProcess Process Plugin

This patch introduces Scripted Processes to lldb.

The goal, here, is to be able to attach in the debugger to fake processes
that are backed by script files (in Python, Lua, Swift, etc ...) and
inspect them statically.

Scripted Processes can be used in cooperative multithreading environments
like the XNU Kernel or other real-time operating systems, but it can
also help us improve the debugger testing infrastructure by writting
synthetic tests that simulates hard-to-reproduce process/thread states.

Although ScriptedProcess is not feature-complete at the moment, it has
basic execution capabilities and will improve in the following patches.

rdar://65508855

Differential Revision: https://reviews.llvm.org/D100384

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

lldb/examples/python/scripted_process/scripted_process.py

@@ -137,6 +137,24 @@ class ScriptedProcess:
         """
         return lldb.SBError()
 
+    @abstractmethod
+    def should_stop(self):
+        """ Check if the scripted process plugin should produce the stop event.
+
+        Returns:
+            bool: True if scripted process should broadcast a stop event.
+                  False otherwise.
+        """
+        pass
+
+    def stop(self):
+        """ Trigger the scripted process stop.
+
+        Returns:
+            lldb.SBError: An `lldb.SBError` with error code 0.
+        """
+        return lldb.SBError()
+
     @abstractmethod
     def is_alive(self):
         """ Check if the scripted process is alive.