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/llvm/raw_os_ostream.h

@@ -1,42 +0,0 @@
-//===- raw_os_ostream.h - std::ostream adaptor for raw_ostream --*- C++ -*-===//
-//
-//                     The LLVM Compiler Infrastructure
-//
-// This file is distributed under the University of Illinois Open Source
-// License. See LICENSE.TXT for details.
-//
-//===----------------------------------------------------------------------===//
-//
-//  This file defines the raw_os_ostream class.
-//
-//===----------------------------------------------------------------------===//
-
-#ifndef LLVM_SUPPORT_RAW_OS_OSTREAM_H
-#define LLVM_SUPPORT_RAW_OS_OSTREAM_H
-
-#include "llvm/raw_ostream.h"
-#include <iosfwd>
-
-namespace llvm {
-
-/// raw_os_ostream - A raw_ostream that writes to an std::ostream.  This is a
-/// simple adaptor class.  It does not check for output errors; clients should
-/// use the underlying stream to detect errors.
-class raw_os_ostream : public raw_ostream {
-  std::ostream &OS;
-
-  /// write_impl - See raw_ostream::write_impl.
-  void write_impl(const char *Ptr, size_t Size) override;
-
-  /// current_pos - Return the current position within the stream, not
-  /// counting the bytes currently in the buffer.
-  uint64_t current_pos() const override;
-
-public:
-  raw_os_ostream(std::ostream &O) : OS(O) {}
-  ~raw_os_ostream() override;
-};
-
-} // end llvm namespace
-
-#endif