Add Clock subscription type to WASIAbi

The change declares a clock subscription type in according with the WebAssembly System Interface standard ABI, which is required for testing clocks and timers in Embedded Swift for WebAssembly.

The ABI for this type is specified publicly in corresponding ABI documentation: https://github.com/WebAssembly/WASI/blob/main/legacy/preview1/docs.md#subscription_clock

rdar://149935761

Author: MaxDesiatov

Sources/WASI/WASI.swift

@@ -429,6 +429,48 @@ enum WASIAbi {
         case END = 2
     }
 
+    struct Clock: GuestPointee {
+        struct Flags: OptionSet, GuestPrimitivePointee {
+            let rawValue: UInt16
+
+            static let isAbsoluteTime = Self(rawValue: 1)
+        }
+
+        let id: ClockId
+        let timeout: Timestamp
+        let precision: Timestamp
+        let flags: Flags
+
+        static let sizeInGuest: UInt32 = 32
+        static let alignInGuest: UInt32 = max(ClockId.alignInGuest, Timestamp.alignInGuest, Flags.alignInGuest)
+
+        static func readFromGuest(_ pointer: UnsafeGuestRawPointer) -> Self {
+            var pointer = pointer
+            return .init(
+                id: .readFromGuest(&pointer),
+                timeout: .readFromGuest(&pointer),
+                precision: .readFromGuest(&pointer),
+                flags: .readFromGuest(&pointer)
+            )
+        }
+
+        static func writeToGuest(at pointer: UnsafeGuestRawPointer, value: Self) {
+            var pointer = pointer
+            ClockId.writeToGuest(at: &pointer, value: value.id)
+            Timestamp.writeToGuest(at: &pointer, value: value.timeout)
+            Timestamp.writeToGuest(at: &pointer, value: value.precision)
+            Flags.writeToGuest(at: &pointer, value: value.flags)
+        }
+    }
+
+    enum EventType: UInt8 {
+        case clock
+        case fdRead
+        case fdWrite
+    }
+
+    typealias UserData = UInt64
+
     enum ClockId: UInt32 {
         /// The clock measuring real time. Time value zero corresponds with
         /// 1970-01-01T00:00:00Z.