Profiler: Use sequential serial numbers for profiling events

Previously Profiler was using timestamps to distinguish processes.
However it is possible that separate processes with the same PID exist
at the exact same timestamp (e.g. for execve). This changes Profiler
to use unique serial numbers for each event instead.

Userland/DevTools/Profiler/EventSerialNumber.h

@@ -0,0 +1,70 @@
+/*
+ * Copyright (c) 2021, Gunnar Beutner <gbeutner@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Assertions.h>
+#include <AK/NumericLimits.h>
+#include <AK/Types.h>
+
+namespace Profiler {
+
+// DistinctNumeric's constructor is non-explicit which makes accidentally turning
+// an unrelated u64 into a serial number all too easy.
+class EventSerialNumber {
+public:
+    constexpr EventSerialNumber() = default;
+
+    void increment()
+    {
+        m_serial++;
+    }
+
+    size_t to_number() const
+    {
+        return m_serial;
+    }
+
+    static EventSerialNumber max_valid_serial()
+    {
+        return EventSerialNumber { NumericLimits<size_t>::max() };
+    }
+
+    bool operator==(EventSerialNumber const& rhs) const
+    {
+        return m_serial == rhs.m_serial;
+    }
+
+    bool operator<(EventSerialNumber const& rhs) const
+    {
+        return m_serial < rhs.m_serial;
+    }
+
+    bool operator>(EventSerialNumber const& rhs) const
+    {
+        return m_serial > rhs.m_serial;
+    }
+
+    bool operator<=(EventSerialNumber const& rhs) const
+    {
+        return m_serial <= rhs.m_serial;
+    }
+
+    bool operator>=(EventSerialNumber const& rhs) const
+    {
+        return m_serial >= rhs.m_serial;
+    }
+
+private:
+    size_t m_serial { 0 };
+
+    constexpr explicit EventSerialNumber(size_t serial)
+        : m_serial(serial)
+    {
+    }
+};
+
+}