PR #18293: [XLA/AOT] Build LLVM with Bazel on Windows

I previously use hacky BUILD file to import prebuilt LLVM binaries from CMake to Bazel so that I can build `tfcompile` with Bazel on Windows, but since Tensorflow updates the LLVM revision used very frequently, this method is too troublesome.

I am trying to make XLA/AOT to work out-of-the-box on Windows with Bazel, then convince Tensorflow team to create a presubmit so that no one can break XLA/AOT on Windows anymore.

Summary:

- Remove unused `llvm_host_triple`.
- Rename `cmake_vars` to `posix_cmake_vars` and create a new `win32_cmake_vars` as Windows is sufficiently different from other platforms.
- Create `LLVM_DEFINES`, `LLVM_COPTS` and `LLVM_LINKOPTS` in `llvm.bzl` to set global compile/link flags for LLVM.
- Create macro `llvm_cc_library` that wrap around `cc_library` to ensure that all targets receive `LLVM_COPTS` consistently. Replace all `cc_library` with `llvm_cc_library`.
- For `support` library, include `lib/Support/Windows/*` for Windows and `lib/Support/Unix/*` for others.

 If `llvm.BUILD` is not to be edited manually no matter what, can the script used to generate `llvm.BUILD` be open-sourced?

#15213

/cc @tatatodd

Imported from GitHub PR #18293

Copybara import of the project:

  - a5e7b841d79878b12e0d0103e5b1813384a80aa8 [XLA/AOT] Build LLVM with Bazel on Windows by Loo Rong Jie <loorongjie@gmail.com>
  - b3a20c7bbff95fd8ad9f44c653a3ecffc54de3b3 Add back LLVM_ENABLE_STATS by Loo Rong Jie <loorongjie@gmail.com>
  - 787d451b495180f5c882d09d76222c157102f0fc Merge b3a20c7bbff95fd8ad9f44c653a3ecffc54de3b3 into 6a1ff... by Loo Rong Jie <loorongjie@gmail.com>

COPYBARA_INTEGRATE_REVIEW=https://github.com/tensorflow/tensorflow/pull/18293 from rongjiecomputer:llvm b3a20c7bbff95fd8ad9f44c653a3ecffc54de3b3
PiperOrigin-RevId: 208073601