Move priority adjustments for async task creation into the runtime.

We're not actually performing the adjustments at the moment due to an
unrelated bug, and will want to perform them within
`swift_task_create_common` based on inheritContext and the given
priority.

Author: DougGregor

stdlib/public/Concurrency/Task.swift

@@ -541,7 +541,7 @@ extension Task where Failure == Never {
 #if compiler(>=5.5) && $BuiltinCreateAsyncTaskInGroup
     // Set up the job flags for a new task.
     var flags = TaskCreateFlags()
-    flags.priority = priority ?? Task<Never, Never>.currentPriority._downgradeUserInteractive
+    flags.priority = priority
     flags.inheritContext = true
     flags.copyTaskLocals = true
     flags.enqueueJob = true
@@ -578,7 +578,7 @@ extension Task where Failure == Error {
 #if compiler(>=5.5) && $BuiltinCreateAsyncTaskInGroup
     // Set up the job flags for a new task.
     var flags = TaskCreateFlags()
-    flags.priority = priority ?? Task<Never, Never>.currentPriority._downgradeUserInteractive
+    flags.priority = priority
     flags.inheritContext = true
     flags.copyTaskLocals = true
     flags.enqueueJob = true
@@ -634,7 +634,7 @@ extension Task where Failure == Never {
 #if compiler(>=5.5) && $BuiltinCreateAsyncTaskInGroup
     // Set up the job flags for a new task.
     var flags = TaskCreateFlags()
-    flags.priority = priority ?? .unspecified
+    flags.priority = priority
     flags.enqueueJob = true
 
     // Create the asynchronous task future.
@@ -689,7 +689,7 @@ extension Task where Failure == Error {
 #if compiler(>=5.5) && $BuiltinCreateAsyncTaskInGroup
     // Set up the job flags for a new task.
     var flags = TaskCreateFlags()
-    flags.priority = priority ?? .unspecified
+    flags.priority = priority
     flags.enqueueJob = true
 
     // Create the asynchronous task future.