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/Base64.h

@@ -0,0 +1,36 @@
+/*----------------------------------------------------------------------------*/
+/* 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_BASE64_H_
+#define WPIUTIL_SUPPORT_BASE64_H_
+
+#include <cstddef>
+#include <string>
+
+#include "wpi/StringRef.h"
+
+namespace wpi {
+template <typename T>
+class SmallVectorImpl;
+class raw_ostream;
+
+size_t Base64Decode(raw_ostream& os, StringRef encoded);
+
+size_t Base64Decode(StringRef encoded, std::string* plain);
+
+StringRef Base64Decode(StringRef encoded, size_t* num_read,
+                       SmallVectorImpl<char>& buf);
+
+void Base64Encode(raw_ostream& os, StringRef plain);
+
+void Base64Encode(StringRef plain, std::string* encoded);
+
+StringRef Base64Encode(StringRef plain, SmallVectorImpl<char>& buf);
+
+}  // namespace wpi
+
+#endif  // WPIUTIL_SUPPORT_BASE64_H_