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/UidVector.h

@@ -0,0 +1,67 @@
+/*----------------------------------------------------------------------------*/
+/* Copyright (c) 2017-2018 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_UIDVECTOR_H_
+#define WPIUTIL_SUPPORT_UIDVECTOR_H_
+
+#include <utility>
+#include <vector>
+
+namespace wpi {
+
+// Vector which provides an integrated freelist for removal and reuse of
+// individual elements.
+// @tparam T element type; must be default-constructible and evaluate in
+//           boolean context to false when "empty"
+// @tparam reuse_threshold how many free elements to store up before starting
+//                         to recycle them
+template <typename T, typename std::vector<T>::size_type reuse_threshold>
+class UidVector {
+ public:
+  typedef typename std::vector<T>::size_type size_type;
+
+  bool empty() const { return m_active_count == 0; }
+  size_type size() const { return m_vector.size(); }
+  T& operator[](size_type i) { return m_vector[i]; }
+  const T& operator[](size_type i) const { return m_vector[i]; }
+
+  // Add a new T to the vector.  If there are elements on the freelist,
+  // reuses the last one; otherwise adds to the end of the vector.
+  // Returns the resulting element index.
+  template <class... Args>
+  size_type emplace_back(Args&&... args) {
+    size_type uid;
+    if (m_free.size() < reuse_threshold) {
+      uid = m_vector.size();
+      m_vector.emplace_back(std::forward<Args>(args)...);
+    } else {
+      uid = m_free.front();
+      m_free.erase(m_free.begin());
+      m_vector[uid] = T(std::forward<Args>(args)...);
+    }
+    ++m_active_count;
+    return uid;
+  }
+
+  // Removes the identified element by replacing it with a default-constructed
+  // one.  The element is added to the freelist for later reuse.
+  void erase(size_type uid) {
+    if (uid >= m_vector.size() || !m_vector[uid]) return;
+    m_free.push_back(uid);
+    m_vector[uid] = T();
+    --m_active_count;
+  }
+
+ private:
+  std::vector<T> m_vector;
+  std::vector<size_type> m_free;
+  size_type m_active_count{0};
+};
+
+}  // namespace wpi
+
+#endif  // WPIUTIL_SUPPORT_UIDVECTOR_H_