Common/Timer: Add a SteadyAwakeClock class which counts non-suspended system running time.

Source/Core/Common/Timer.cpp

@@ -5,6 +5,7 @@
 
 #include <chrono>
 #include <thread>
+#include "Common/CommonFuncs.h"
 
 #ifdef _WIN32
 #include <Windows.h>
@@ -194,4 +195,37 @@ void PrecisionTimer::SleepUntil(Clock::time_point target)
   }
 }
 
+// Results are appropriately slewed on Linux, but not on Windows, macOS, or FreeBSD.
+// Clocks with that functionality seem to not be available there.
+auto SteadyAwakeClock::now() -> time_point
+{
+#if defined(_WIN32)
+  // The count is system time "in units of 100 nanoseconds".
+  using InterruptDuration = std::chrono::duration<ULONGLONG, std::ratio<100, std::nano::den>::type>;
+
+  ULONGLONG interrupt_time{};
+  if (!QueryUnbiasedInterruptTime(&interrupt_time))
+    ERROR_LOG_FMT(COMMON, "QueryUnbiasedInterruptTime");
+
+  return time_point{InterruptDuration{interrupt_time}};
+#else
+  // Note that Linux's CLOCK_MONOTONIC "does not count time that the system is suspended".
+  // This is in contrast to the behavior on macOS and FreeBSD.
+  static constexpr auto clock_id =
+#if defined(__linux__)
+      CLOCK_MONOTONIC;
+#elif defined(__APPLE__)
+      CLOCK_UPTIME_RAW;
+#else
+      CLOCK_UPTIME;
+#endif
+
+  timespec ts{};
+  if (clock_gettime(clock_id, &ts) != 0)
+    ERROR_LOG_FMT(COMMON, "clock_gettime: {}", LastStrerrorString());
+
+  return time_point{std::chrono::seconds{ts.tv_sec} + std::chrono::nanoseconds{ts.tv_nsec}};
+#endif
+}
+
 }  // Namespace Common

Source/Core/Common/Timer.h

@@ -50,4 +50,19 @@ private:
 #endif
 };
 
+// Similar to std::chrono::steady_clock except this clock
+// specifically does *not* count time while the system is suspended.
+class SteadyAwakeClock
+{
+public:
+  using rep = s64;
+  using period = std::nano;
+  using duration = std::chrono::duration<rep, period>;
+  using time_point = std::chrono::time_point<SteadyAwakeClock>;
+
+  static constexpr bool is_steady = true;
+
+  static time_point now();
+};
+
 }  // Namespace Common