[NFC] Update dispatch test for new preconcurrency annotations.

(cherry picked from commit 2d71d91)

Author: hborla

test/Concurrency/dispatch_inference.swift

@@ -22,15 +22,15 @@ func testMe() {
 func testUnsafeSendableInMainAsync() async {
   var x = 5
   DispatchQueue.main.async {
-    x = 17 // expected-error{{mutation of captured var 'x' in concurrently-executing code}}
+    x = 17 // expected-warning{{mutation of captured var 'x' in concurrently-executing code}}
   }
   print(x)
 }
 
 func testUnsafeSendableInAsync(queue: DispatchQueue) async {
   var x = 5
   queue.async {
-    x = 17 // expected-error{{mutation of captured var 'x' in concurrently-executing code}}
+    x = 17 // expected-warning{{mutation of captured var 'x' in concurrently-executing code}}
   }
 
   queue.sync {