[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

lldb/include/lldb/Target/Process.h

@@ -364,7 +364,6 @@ inline bool operator!=(const ProcessModID &lhs, const ProcessModID &rhs) {
 /// A plug-in interface definition class for debugging a process.
 class Process : public std::enable_shared_from_this<Process>,
                 public ProcessProperties,
-                public UserID,
                 public Broadcaster,
                 public ExecutionContextScope,
                 public PluginInterface {
@@ -560,6 +559,15 @@ public:
 
   uint32_t GetAddressByteSize() const;
 
+  /// Sets the stored pid.
+  ///
+  /// This does not change the pid of underlying process.
+  lldb::pid_t GetID() const { return m_pid; }
+
+  /// Returns the pid of the process or LLDB_INVALID_PROCESS_ID if there is
+  /// no known pid.
+  void SetID(lldb::pid_t new_pid) { m_pid = new_pid; }
+
   uint32_t GetUniqueID() const { return m_process_unique_id; }
 
   /// Check if a plug-in instance can debug the file in \a module.
@@ -2730,6 +2738,7 @@ protected:
 
   // Member variables
   std::weak_ptr<Target> m_target_wp; ///< The target that owns this process.
+  lldb::pid_t m_pid = LLDB_INVALID_PROCESS_ID;
   ThreadSafeValue<lldb::StateType> m_public_state;
   ThreadSafeValue<lldb::StateType>
       m_private_state;                     // The actual state of our process

lldb/source/Target/Process.cpp

@@ -529,7 +529,7 @@ Process::Process(lldb::TargetSP target_sp, ListenerSP listener_sp)
 
 Process::Process(lldb::TargetSP target_sp, ListenerSP listener_sp,
                  const UnixSignalsSP &unix_signals_sp)
-    : ProcessProperties(this), UserID(LLDB_INVALID_PROCESS_ID),
+    : ProcessProperties(this),
       Broadcaster((target_sp->GetDebugger().GetBroadcasterManager()),
                   Process::GetStaticBroadcasterClass().AsCString()),
       m_target_wp(target_sp), m_public_state(eStateUnloaded),