[TSan] Deflake positive TSan+libdispatch test

TSan does not guarantee the detection of races [1].  This means that any
positive test, i.e., a test that asserts the detection of a race, comes
with inherent flakiness.  In the LLVM TSan test suite this unfortunate
circumstance is mitigated via the deflake utility [2] (plus lit
substitution) which executes a test multiple times.

Since we currently only have 3 positive TSan tests in the Swift test
suite [3] (the remaining ones test the absence of false positives), we
decided against bringing over this utility for now. Instead this change
attempts to "deflake" as part of the test itself.

[1] https://groups.google.com/d/msg/thread-sanitizer/KIok3F_b1oI/RwrMNhyHafwJ
[2] llvm/llvm-project@233f401
[3] Positive TSan tests:
  test/Sanitizers/tsan.swift
  test/Sanitizers/tsan-libdispatch.swift
  validation-test/Sanitizers/tsan-inout.swift

rdar://51730684

Author: yln

test/Sanitizers/tsan-libdispatch.swift

@@ -9,36 +9,43 @@
 // don't support TSan.
 // UNSUPPORTED: remote_run
 
-// rdar://51730684
-// REQUIRES: disable
-
 // Test ThreadSanitizer execution end-to-end with libdispatch.
 
 import Dispatch
 
+let q = DispatchQueue(label: "q", attributes: .concurrent)
+
 let sync1 = DispatchSemaphore(value: 0)
 let sync2 = DispatchSemaphore(value: 0)
 let finish = DispatchSemaphore(value: 0)
 
-let q = DispatchQueue(label: "q", attributes: .concurrent)
-
-var racy = 1
-
-q.async {
-  sync1.wait()
-  sync2.signal()
-  racy = 2
-  finish.signal()
-}
-q.async {
-  sync1.signal()
-  sync2.wait()
-  racy = 3
-  finish.signal()
+func wait1_signal2() { sync1.wait();   sync2.signal() }
+func signal1_wait2() { sync1.signal(); sync2.wait()   }
+
+func race() {
+  var racy = 1
+
+  q.async {
+    wait1_signal2()
+    racy = 2
+    wait1_signal2()
+    finish.signal()
+  }
+  q.async {
+    signal1_wait2()
+    racy = 3
+    signal1_wait2()
+    finish.signal()
+  }
+
+  finish.wait()
+  finish.wait()
 }
 
-finish.wait()
-finish.wait()
+// TSan %deflake as part of the test.
+for _ in 1...10 {
+  race()
+}
 
 print("Done!")
 

test/Sanitizers/tsan.swift

@@ -44,6 +44,7 @@ public func call_foobar() {
 var threads: [pthread_t] = []
 var racey_x: Int;
 
+// TSan %deflake as part of the test.
 for _ in 1...5 {
 #if os(macOS) || os(iOS)
   var t : pthread_t?