[Concurrency] An implementation of system epochs for continuous and suspending clocks (swiftlang#80409)

This implements
[SE-0473](https://github.com/swiftlang/swift-evolution/blob/main/proposals/0473-clock-epochs.md)

Author: phausler

stdlib/public/Concurrency/ContinuousClock.swift

@@ -139,6 +139,14 @@ extension ContinuousClock: Clock {
 #endif
 }
 
+@available(SwiftStdlib 5.7, *)
+@_unavailableInEmbedded
+extension ContinuousClock {
+  @available(SwiftStdlib 5.7, *)
+  @_alwaysEmitIntoClient
+  public var systemEpoch: Instant { unsafeBitCast(Duration.seconds(0), to: Instant.self) }
+}
+
 @available(SwiftStdlib 5.7, *)
 @_unavailableInEmbedded
 extension ContinuousClock.Instant: InstantProtocol {

stdlib/public/Concurrency/SuspendingClock.swift

@@ -127,6 +127,14 @@ extension SuspendingClock: Clock {
 #endif
 }
 
+@available(SwiftStdlib 5.7, *)
+@_unavailableInEmbedded
+extension SuspendingClock {
+  @available(SwiftStdlib 5.7, *)
+  @_alwaysEmitIntoClient
+  public var systemEpoch: Instant { unsafeBitCast(Duration.seconds(0), to: Instant.self) }
+}
+
 @available(SwiftStdlib 5.7, *)
 @_unavailableInEmbedded
 extension SuspendingClock.Instant: InstantProtocol {

test/Concurrency/Runtime/clock.swift

@@ -158,6 +158,13 @@ import StdlibUnittest
       }
     }
 
+    tests.test("Ensure abi layout size of Instant") {
+      // If this test fails it means the ABI of ContinuousClock.Instant has been broken!
+      // it MUST be the same laoyut of that of Duration
+      expectEqual(MemoryLayout<ContinuousClock.Instant>.size, MemoryLayout<Duration>.size)
+      expectEqual(MemoryLayout<SuspendingClock.Instant>.size, MemoryLayout<Duration>.size)
+    }
+
     await runAllTestsAsync()
   }
 }

test/abi/macOS/arm64/concurrency.swift

@@ -418,6 +418,10 @@ Added: _swift_task_dealloc_through
 // SwiftSettings
 Added: _$ss12SwiftSettingVsE16defaultIsolationyABScA_pXpSgFZ
 
+// Clock systemEpochs
+Added: _$ss15ContinuousClockV11systemEpochAB7InstantVvpMV
+Added: _$ss15SuspendingClockV11systemEpochAB7InstantVvpMV
+
 // Hashable for (Throwing)AsyncStream
 Added: _$sScS12ContinuationV7storageScS8_StorageCyx_Gvg
 Added: _$sScS12ContinuationV7storageScS8_StorageCyx_GvpMV
@@ -434,4 +438,4 @@ Added: _$sScs8_StorageCMn
 Added: _$sScs8_StorageCMo
 Added: _$sScs8_StorageCMu
 Added: _$sScs8_StorageCfD
-Added: _$sScs8_StorageCfd
+Added: _$sScs8_StorageCfd

test/abi/macOS/x86_64/concurrency.swift

@@ -418,6 +418,10 @@ Added: _swift_task_dealloc_through
 // SwiftSettings
 Added: _$ss12SwiftSettingVsE16defaultIsolationyABScA_pXpSgFZ
 
+// Clock systemEpochs
+Added: _$ss15ContinuousClockV11systemEpochAB7InstantVvpMV
+Added: _$ss15SuspendingClockV11systemEpochAB7InstantVvpMV
+
 // Hashable for (Throwing)AsyncStream
 Added: _$sScS12ContinuationV7storageScS8_StorageCyx_Gvg
 Added: _$sScS12ContinuationV7storageScS8_StorageCyx_GvpMV
@@ -434,4 +438,4 @@ Added: _$sScs8_StorageCMn
 Added: _$sScs8_StorageCMo
 Added: _$sScs8_StorageCMu
 Added: _$sScs8_StorageCfD
-Added: _$sScs8_StorageCfd
+Added: _$sScs8_StorageCfd