[lldb][NFC] Don't let Process inherit from UserID
I noticed that Process is inheriting from UserID to store its PID value. This patch
replaces this with a dedicated field in the Process class. This is NFC, but has some
small effects on the code using Process:
- GetID() now returns a lldb::pid_t like all other process code instead of lldb::user_id_t. Both are typedefs for uint64_t, so no change in behaviour.
- The equality operators defined for UserID no longer accept Process instances.
- Removes the inherited method Process::Clear() which didn't actually clear anything beside the PID value.
We maybe should also remove the getters/setters to S/GetPID or something like that. I can update all the code for that
in a follow-up NFC commit.
Reviewed By: labath
Differential Revision: https://reviews.llvm.org/D91699