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/support/sha1.h

@@ -1,56 +0,0 @@
-/*
-    sha1.hpp - header of
-
-    ============
-    SHA-1 in C++
-    ============
-
-    100% Public Domain.
-
-    Original C Code
-        -- Steve Reid <steve@edmweb.com>
-    Small changes to fit into bglibs
-        -- Bruce Guenter <bruce@untroubled.org>
-    Translation to simpler C++ Code
-        -- Volker Grabsch <vog@notjusthosting.com>
-    Safety fixes
-        -- Eugene Hopkinson <slowriot at voxelstorm dot com>
-*/
-
-#ifndef WPIUTIL_SUPPORT_SHA1_H_
-#define WPIUTIL_SUPPORT_SHA1_H_
-
-#include <stdint.h>
-
-#include <string>
-
-#include "llvm/StringRef.h"
-
-namespace llvm {
-template <typename T>
-class SmallVectorImpl;
-}  // namespace llvm
-
-namespace wpi {
-
-class raw_istream;
-
-class SHA1 {
- public:
-  SHA1();
-  void Update(llvm::StringRef s);
-  void Update(raw_istream& is);
-  std::string Final();
-  llvm::StringRef Final(llvm::SmallVectorImpl<char>& buf);
-  static std::string FromFile(llvm::StringRef filename);
-
- private:
-  uint32_t digest[5];
-  unsigned char buffer[64];
-  size_t buf_size;
-  uint64_t transforms;
-};
-
-}  // namespace wpi
-
-#endif  // WPIUTIL_SUPPORT_SHA1_H_