Move entirety of llvm namespace to wpi namespace.

During shared library loading, a different libLLVM can be pulled in, causing
llvm symbols from dependent libraries to resolve to that library instead of
this one. This has been seen in the wild with the Mesa OpenGL implementation
in JavaFX applications (see wpilibsuite/shuffleboard#361).

This is clearly a very breaking change. For some level of backwards
compatibility, a namespace alias from llvm to wpi is performed in the "llvm"
headers.  Unfortunately, forward declarations of llvm classes will still break,
but compilers seem to generate clear error messages in those cases
("namespace alias 'llvm' not allowed here, assuming 'wpi'").

This change also moves all the wpiutil headers to a single "wpi" subdirectory
from the previously split "llvm", "support", "tcpsockets", and "udpsockets".
Shim headers will be added for backwards compatibility in a later commit.

wpiutil/src/main/native/include/wpi/SafeThread.h

@@ -0,0 +1,133 @@
+/*----------------------------------------------------------------------------*/
+/* Copyright (c) 2015-2018 FIRST. All Rights Reserved.                        */
+/* Open Source Software - may be modified and shared by FRC teams. The code   */
+/* must be accompanied by the FIRST BSD license file in the root directory of */
+/* the project.                                                               */
+/*----------------------------------------------------------------------------*/
+
+#ifndef WPIUTIL_SUPPORT_SAFETHREAD_H_
+#define WPIUTIL_SUPPORT_SAFETHREAD_H_
+
+#include <atomic>
+#include <thread>
+
+#include "wpi/condition_variable.h"
+#include "wpi/mutex.h"
+
+namespace wpi {
+
+// Base class for SafeThreadOwner threads.
+class SafeThread {
+ public:
+  SafeThread() { m_active = true; }
+  virtual ~SafeThread() = default;
+  virtual void Main() = 0;
+
+  wpi::mutex m_mutex;
+  std::atomic_bool m_active;
+  wpi::condition_variable m_cond;
+};
+
+namespace detail {
+
+// Non-template proxy base class for common proxy code.
+class SafeThreadProxyBase {
+ public:
+  explicit SafeThreadProxyBase(SafeThread* thr) : m_thread(thr) {
+    if (!m_thread) return;
+    m_lock = std::unique_lock<wpi::mutex>(m_thread->m_mutex);
+    if (!m_thread->m_active) {
+      m_lock.unlock();
+      m_thread = nullptr;
+      return;
+    }
+  }
+  explicit operator bool() const { return m_thread != nullptr; }
+  std::unique_lock<wpi::mutex>& GetLock() { return m_lock; }
+
+ protected:
+  SafeThread* m_thread;
+  std::unique_lock<wpi::mutex> m_lock;
+};
+
+// A proxy for SafeThread.
+// Also serves as a scoped lock on SafeThread::m_mutex.
+template <typename T>
+class SafeThreadProxy : public SafeThreadProxyBase {
+ public:
+  explicit SafeThreadProxy(SafeThread* thr) : SafeThreadProxyBase(thr) {}
+  T& operator*() const { return *static_cast<T*>(m_thread); }
+  T* operator->() const { return static_cast<T*>(m_thread); }
+};
+
+// Non-template owner base class for common owner code.
+class SafeThreadOwnerBase {
+ public:
+  void Stop();
+
+  SafeThreadOwnerBase() { m_thread = nullptr; }
+  SafeThreadOwnerBase(const SafeThreadOwnerBase&) = delete;
+  SafeThreadOwnerBase& operator=(const SafeThreadOwnerBase&) = delete;
+  SafeThreadOwnerBase(SafeThreadOwnerBase&& other)
+      : m_thread(other.m_thread.exchange(nullptr)) {}
+  SafeThreadOwnerBase& operator=(SafeThreadOwnerBase other) {
+    SafeThread* otherthr = other.m_thread.exchange(nullptr);
+    SafeThread* curthr = m_thread.exchange(otherthr);
+    other.m_thread.exchange(curthr);  // other destructor will clean up
+    return *this;
+  }
+  ~SafeThreadOwnerBase() { Stop(); }
+
+  explicit operator bool() const { return m_thread.load(); }
+
+ protected:
+  void Start(SafeThread* thr);
+  SafeThread* GetThread() const { return m_thread.load(); }
+  std::thread::native_handle_type GetNativeThreadHandle() const {
+    return m_nativeHandle;
+  }
+
+ private:
+  std::atomic<SafeThread*> m_thread;
+  std::atomic<std::thread::native_handle_type> m_nativeHandle;
+};
+
+inline void SafeThreadOwnerBase::Start(SafeThread* thr) {
+  SafeThread* curthr = nullptr;
+  SafeThread* newthr = thr;
+  if (!m_thread.compare_exchange_strong(curthr, newthr)) {
+    delete newthr;
+    return;
+  }
+  std::thread stdThread([=]() {
+    newthr->Main();
+    delete newthr;
+  });
+  m_nativeHandle = stdThread.native_handle();
+  stdThread.detach();
+}
+
+inline void SafeThreadOwnerBase::Stop() {
+  SafeThread* thr = m_thread.exchange(nullptr);
+  if (!thr) return;
+  thr->m_active = false;
+  thr->m_cond.notify_one();
+}
+
+}  // namespace detail
+
+template <typename T>
+class SafeThreadOwner : public detail::SafeThreadOwnerBase {
+ public:
+  void Start() { Start(new T); }
+  void Start(T* thr) { detail::SafeThreadOwnerBase::Start(thr); }
+
+  using Proxy = typename detail::SafeThreadProxy<T>;
+  Proxy GetThread() const {
+    return Proxy(detail::SafeThreadOwnerBase::GetThread());
+  }
+};
+
+}  // namespace wpi
+
+#endif  // WPIUTIL_SUPPORT_SAFETHREAD_H_