[Backtrace] show the crashed thread first in standard crash logs

It can be hard to find the crashed thread. This always shows it first, before
any other threads.

rdar://164566321

stdlib/public/libexec/swift-backtrace/main.swift

@@ -894,12 +894,11 @@ Generate a backtrace for the parent process.
       }
     }
 
+    dump(ndx: target.crashingThreadNdx, thread: crashingThread)
     if args.threads! {
-      for (ndx, thread) in target.threads.enumerated() {
+      for (ndx, thread) in target.threads.enumerated() where ndx != target.crashingThreadNdx {
         dump(ndx: ndx, thread: thread)
       }
-    } else {
-      dump(ndx: target.crashingThreadNdx, thread: crashingThread)
     }
 
     if args.registers! == .crashedOnly {

test/Backtracing/CrashWithThreads.swift

@@ -0,0 +1,85 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-build-swift %s -Onone -g -o %t/CrashWithThreads
+// RUN: %target-codesign %t/CrashWithThreads
+// RUN: (env SWIFT_BACKTRACE=enable=yes,cache=no,swift-backtrace=%backtracer %target-run %t/CrashWithThreads 2>&1 || true) | %FileCheck -vv %s -dump-input-filter=all
+
+// UNSUPPORTED: use_os_stdlib
+// UNSUPPORTED: back_deployment_runtime
+// UNSUPPORTED: asan
+// REQUIRES: executable_test
+// REQUIRES: backtracing
+// REQUIRES: OS=macosx || OS=linux-gnu
+
+#if canImport(Darwin)
+  import Darwin
+#elseif canImport(Glibc)
+  import Glibc
+#elseif canImport(Android)
+  import Android
+#elseif os(Windows)
+  import CRT
+#else
+#error("Unsupported platform")
+#endif
+
+func reallyCrashMe() {
+  print("I'm going to crash now")
+  let ptr = UnsafeMutablePointer<Int>(bitPattern: 4)!
+  ptr.pointee = 42
+}
+
+func crashMe() {
+  reallyCrashMe()
+}
+
+func spawnThread(_ shouldCrash: Bool) {
+  #if os(Linux)
+  var thread: pthread_t = 0
+  #elseif os(macOS)
+  var thread = pthread_t(nil)
+  #endif
+  if shouldCrash {
+    pthread_create(&thread, nil, { _ in
+                                crashMe()
+                                // this should not be run
+                                while (true) {
+                                    sleep(10)
+                                  }
+                              }, nil)
+  } else {
+    pthread_create(&thread, nil, { _ in
+                                  while (true) {
+                                    sleep(10)
+                                  }
+                                }, nil)
+  }
+}
+
+let crashingThreadIndex = (1..<10).randomElement()
+
+for threadIndex in 0..<10 {
+  spawnThread(threadIndex == crashingThreadIndex)
+}
+
+while (true) {
+  sleep(10)
+}
+
+// CHECK: *** Program crashed: Bad pointer dereference at 0x{{0+}}4 ***
+
+// make sure there are no threads before the crashing thread (rdar://164566321)
+
+// CHECK-NOT: Thread {{[0-9]+}}:
+
+// CHECK: Thread {{[1-9]+}} {{(".*" )?}}crashed:
+
+// CHECK: 0                    0x{{[0-9a-f]+}} reallyCrashMe() + {{[0-9]+}} in CrashWithThreads at {{.*}}/CrashWithThreads
+// CHECK-NEXT: 1 [ra]          0x{{[0-9a-f]+}} crashMe() + {{[0-9]+}} in CrashWithThreads at {{.*}}/CrashWithThreads
+// CHECK-NEXT: 2 [ra]          0x{{[0-9a-f]+}} closure #{{[0-9]}} in spawnThread(_:) + {{[0-9]+}} in CrashWithThreads at {{.*}}/CrashWithThreads
+// CHECK-NEXT: 3 [ra] [thunk]  0x{{[0-9a-f]+}} @objc closure #{{[0-9]}} in spawnThread(_:) + {{[0-9]+}} in CrashWithThreads at {{.*}}<compiler-generated>
+
+// CHECK: Registers:
+
+// CHECK: Images ({{[0-9]+}} omitted):
+
+// CHECK: {{0x[0-9a-f]+}}–{{0x[0-9a-f]+}}{{ +}}{{([0-9a-f]+|<no build ID>)}}{{ +}}CrashWithThreads{{ +}}{{.*}}/CrashWithThreads