Add a test for the tsan co-routine lowering order change

rdar://48719789

Author: aschwaighofer

test/IRGen/tsan_coroutines.swift

@@ -0,0 +1,27 @@
+// This test case used to crash when tsan ran before co-routine lowering.
+// RUN: %target-swift-frontend -emit-ir -sanitize=thread %s | %FileCheck %s
+
+public class C { }
+
+public struct Foobar {
+  var things: [String: C] = [:]
+}
+
+extension Foobar {
+    public struct Index {
+        fileprivate typealias MyIndex = Dictionary<String, C>.Values.Index
+
+        fileprivate let myIndex: MyIndex
+
+        fileprivate init(_ index: MyIndex) {
+            self.myIndex = index
+        }
+    }
+
+    // We used to crash emitting the subscript function.
+    // CHECK: define swiftcc { i8*, %T15tsan_coroutines1CC* } @"$s15tsan_coroutines6FoobarVyAA1CCAC5IndexVcir"
+    @_borrowed
+    public subscript(position: Index) -> C {
+        return things.values[position.myIndex]
+    }
+}