[Concurrency] Perform userInitiated adjustment after determining priority.

Based on feedback, perform the adjustment from userInteractive to
userInitiated all the time, and rely on `qos_class_self` whenever we
don't have a task.

(cherry picked from commit fba8cca)

Author: DougGregor

stdlib/public/Concurrency/Actor.cpp

@@ -261,12 +261,12 @@ static bool isExecutingOnMainThread() {
 }
 
 JobPriority swift::swift_task_getCurrentThreadPriority() {
-  if (isExecutingOnMainThread())
-    return JobPriority::UserInitiated;
-
 #if defined(__APPLE__)
   return static_cast<JobPriority>(qos_class_self());
 #else
+  if (isExecutingOnMainThread())
+    return JobPriority::UserInitiated;
+
   return JobPriority::Unspecified;
 #endif
 }

stdlib/public/Concurrency/Task.swift

@@ -540,6 +540,10 @@ public func async(
     actualPriority = priority
   }
 
+  let adjustedPriority = actualPriority == .userInteractive
+    ? .userInitiated
+    : actualPriority
+
   // Set up the job flags for a new task.
   var flags = Task.JobFlags()
   flags.kind = .task