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/cpp/llvm/Hashing.cpp

@@ -13,17 +13,17 @@
 //
 //===----------------------------------------------------------------------===//
 
-#include "llvm/Hashing.h"
+#include "wpi/Hashing.h"
 
-using namespace llvm;
+using namespace wpi;
 
 // Provide a definition and static initializer for the fixed seed. This
 // initializer should always be zero to ensure its value can never appear to be
 // non-zero, even during dynamic initialization.
-size_t llvm::hashing::detail::fixed_seed_override = 0;
+size_t wpi::hashing::detail::fixed_seed_override = 0;
 
 // Implement the function for forced setting of the fixed seed.
 // FIXME: Use atomic operations here so that there is no data race.
-void llvm::set_fixed_execution_hash_seed(size_t fixed_value) {
+void wpi::set_fixed_execution_hash_seed(size_t fixed_value) {
   hashing::detail::fixed_seed_override = fixed_value;
 }