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ARROW-3846: [Gandiva][C++] Build Gandiva C++ libraries and get unit tests passing on Windows

The tests pass cleanly for me on Windows with these changes. Can't say this was the most enjoyable project. I will fix up the CI but wanted to get eyes on the changes in case anything looks undesirable

Some notes

* This requires that LLVM was built with CMake, which is not true
  of the Windows installers from llvm.org. It works for me if I
  `conda install llvmdev=6.0.1 clangdev=6.0.1`
* I had to suppress a ton of compiler warnings that seem to be
  coming from the LLVM headers. see gandiva/llvm_includes.h
* Unix targets are still using strptime for date parsing. We
  might want to use `arrow/vendored/date.h` on all platforms
  unless there is some significant performance difference
* This doesn't build in Appveyor yet. I wanted to wait to see which
  builds folks think we should add this too (build times will go up
  because installing llvmdev/clangdev will be time consuming)
* Python or other bindings will have to be investigated in a separate patch

Author: Wes McKinney <wesm+git@apache.org>

Closes #3295 from wesm/gandiva-windows and squashes the following commits:

1fb56714 <Wes McKinney> Add missing file
1e0fdbc8 <Wes McKinney> Revert Flatbuffers changes that are causing CI flakiness
5fb78bc4 <Wes McKinney> Bump flatbuffers version to v1.10.0
707e9d9f <Wes McKinney> Visibility fixes for windows with the cast_time changes
fac669e0 <Wes McKinney> Rebase, remove failing unit test
f3b60bf3 <Wes McKinney> Some basic fixes to build system, clang IR generation, suppress endogenous LLVM warnings with MSVC 2017