Fix caffee2 for llvm trunk

Summary: Fix build with llvm-trunk. With D25877605 (pytorch@cb37709), we need to explicitly include `llvm/Support/Host.h` in `llvm_jit.cpp`.

Test Plan: `buck build mode/opt-clang -j 56 sigrid/predictor/v2:sigrid_remote_predictor -c cxx.extra_cxxflags="-Wforce-no-error" -c cxx.modules=False -c cxx.use_default_autofdo_profile=False`

Reviewed By: bertmaher

Differential Revision: D25920968

fbshipit-source-id: 4b80d5072907f50d01e8fbef41cda8a89dd66a96

torch/csrc/jit/tensorexpr/llvm_jit.cpp

@@ -15,6 +15,7 @@
 #include <llvm/IR/DataLayout.h>
 #include <llvm/IR/Mangler.h>
 #include <llvm/Support/DynamicLibrary.h>
+#include <llvm/Support/Host.h>
 #include <llvm/Support/raw_ostream.h>
 #include <llvm/Target/TargetMachine.h>