Merge pull request swiftlang#80685 from tshortli/async-sequence-existential-dump-output

Tests: Use `-o` instead of stdout redirect in `async_sequence_existential.swift`

Author: tshortli

test/Concurrency/async_sequence_existential.swift

@@ -1,6 +1,7 @@
 // RUN: %target-swift-frontend -target %target-swift-5.1-abi-triple %s -emit-sil -o /dev/null -verify
 
-// RUN: %target-swift-frontend -target %target-swift-5.1-abi-triple %s -dump-ast 2>&1 | %FileCheck %s
+// RUN: %target-swift-frontend -target %target-swift-5.1-abi-triple -primary-file %s -dump-ast -o %t.ast.txt
+// RUN: %FileCheck %s < %t.ast.txt
 
 // REQUIRES: concurrency