[libcxx][SystemZ][z/OS] Update libcxx/src/debug.cpp to accommodate POSIX(OFF)

[DanielMcIntosh-IBM]

Summary

Use a wrapper class around std::mutex to prevent calls to mutex functions when
the threading API is disabled.

Depends on D110349

[jroelofs]

LGTM

libcxx/src/debug.cpp

70

[DanielMcIntosh-IBM]

Address review comments

libcxx/src/debug.cpp

 {
     return __get_db();
 }
 
 namespace
 {
 
 #ifndef _LIBCPP_HAS_NO_THREADS
-typedef mutex mutex_type;
+/// Simple wrapper around std::mutex that checks threads are enabled before doing anything.
+class mutex_check_threading : public mutex {
+    /// Threads enabled/disabled status should not change over the life of the process.
+    static const bool threadsEnabled;
+
+  public:
+    void lock() {
+        if (!threadsEnabled)
+            return;
+        mutex::lock();
+    }
+    bool try_lock() {
+        if (!threadsEnabled)
+            return true;
+        return mutex::try_lock();
+    }
+    void unlock() {
+        if (!threadsEnabled)
+            return;
+        mutex::unlock();
+    }
+};
+const bool mutex_check_threading::threadsEnabled = __libcpp_are_threads_enabled();
+
+typedef mutex_check_threading mutex_type;
 typedef lock_guard<mutex_type> WLock;
 typedef lock_guard<mutex_type> RLock;
 
 mutex_type&
 mut()
 {
     static _LIBCPP_NO_DESTROY mutex_type m;
     return m;