Inherit main thread priority/context

This patch changes the main task to inherit the context of the main
thread. This should assign the appropriate priority based on how the
program was invoked. I've also updated the tests to reflect these
changes.

Author: etcwilde

lib/SILGen/SILGenFunction.cpp

@@ -900,6 +900,7 @@ void SILGenFunction::emitAsyncMainThreadStart(SILDeclRef entryPoint) {
 
   // Emit the CreateAsyncTask builtin
   TaskCreateFlags taskCreationFlagMask(0);
+  taskCreationFlagMask.setInheritContext(true);
   SILValue taskFlags =
       emitWrapIntegerLiteral(moduleLoc, getLoweredType(ctx.getIntType()),
                              taskCreationFlagMask.getOpaqueValue());

test/Concurrency/Runtime/async_task_priority_current.swift

@@ -20,7 +20,7 @@ extension TaskPriority: CustomStringConvertible {
 @available(SwiftStdlib 5.5, *)
 func test_detach() async {
   let a1 = Task.currentPriority
-  print("a1: \(a1)") // CHECK: TaskPriority(rawValue: 21)
+  print("a1: \(a1)") // CHECK: TaskPriority(rawValue: 25)
 
   // Note: remember to detach using a higher priority, otherwise a lower one
   // might be escalated by the get() and we could see `default` in the detached
@@ -31,7 +31,7 @@ func test_detach() async {
   }.get()
 
   let a3 = Task.currentPriority
-  print("a3: \(a3)") // CHECK: a3: TaskPriority(rawValue: 21)
+  print("a3: \(a3)") // CHECK: a3: TaskPriority(rawValue: 25)
 }
 
 @available(SwiftStdlib 5.5, *)

test/Concurrency/async_main.swift

@@ -56,7 +56,7 @@ func asyncFunc() async {
 
 // CHECK-SIL:       // function_ref async_Main
 // CHECK-SIL-NEXT:  %2 = function_ref @async_Main : $@convention(thin) @async () -> ()
-// CHECK-SIL-NEXT:  %3 = integer_literal $Builtin.Int64, 21
+// CHECK-SIL-NEXT:  %3 = integer_literal $Builtin.Int64, 2048
 // CHECK-SIL-NEXT:  %4 = struct $Int (%3 : $Builtin.Int64)
 // CHECK-SIL-NEXT:  %5 = metatype $@thick ().Type
 // CHECK-SIL-NEXT:  %6 = init_existential_metatype %5 : $@thick ().Type, $@thick Any.Type