[Concurrency] Add tracing for major operations in the concurrency runtime.

Each trace point is declared as a function in the new `Tracing.h` header. These functions are called from the appropriate places in the concurrency runtime.

On Darwin, an implementation of these functions is provided which uses the `os/signpost.h` API to emit signpost events/intervals.

When the signpost API is not available, no-op stub implementations are provided. Implementations for other OSes can be provided by providing implementations of the trace functions for that OS.

rdar://81858487

include/swift/ABI/Executor.h

@@ -124,6 +124,9 @@ public:
   /// Is this executor the main executor?
   bool isMainExecutor() const;
 
+  /// Get the raw value of the Implementation field, for tracing.
+  uintptr_t getRawImplementation() { return Implementation; }
+
   bool operator==(ExecutorRef other) const {
     return Identity == other.Identity;
   }