[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