Fix caffee2 for llvm trunk
Summary: Fix build with llvm-trunk. With D25877605 (https://github.com/pytorch/pytorch/commit/cb37709beeb84a04f4698a2961dab91ef4b6eae9), 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
