Gardening: Migrate test suite to GH issues (file names): Concurrency

Author: AnthonyLatsis

test/Concurrency/sr15049.swift renamed to test/Concurrency/issue-57376.swift

@@ -1,6 +1,8 @@
 // RUN: %target-typecheck-verify-swift -disable-availability-checking -strict-concurrency=targeted
 // REQUIRES: concurrency
 
+// https://github.com/apple/swift/issues/57376
+
 func testAsyncSequenceTypedPatternSendable<Seq: AsyncSequence>(_ seq: Seq) async throws where Seq.Element == Int, Seq: Sendable {
    async let result: Int = seq.reduce(0) { $0 + $1 } // OK
    // expected-warning@-1{{immutable value 'result' was never used; consider replacing with '_' or removing it}}