feat: windows toolchain for running clang tools

[peakschris]

This is a start on windows toolchain support (#4). Enough is in place for the distribution to be downloaded, extracted, and tools made available through symlinks and a build file.

working examples:
bazel run @llvm_toolchain_llvm//:bin/clang-tidy.exe
bazel run @llvm_toolchain_llvm//:bin/clang-format.exe

llvm 18+ is required.

Comments, suggestions, help all very welcome

[rrbutani]

@peakschris Thanks for taking a stab at this.

I'm not super familiar with Windows C/C++ development but I left some notes.

If you haven't come across it already, #154 may be of interest to you; it uses cc_toolchain_config from @bazel_tools//tools/cpp:windows_cc_toolchain_config.bzl which we'll probably also want to eventually do.

[rrbutani]

WDYT about unifying this with BUILD.llvm_repo? It seems like the discrepancies are mostly just in the file extensions (.exe, .lib for Windows).

Maybe something like:

is_windows = %{is_windows} # provided by `llvm_repo`
executable_extension = ".exe" if is_windows else ""

filegroup(
  name = "clang",
  srcs = [
    "bin/" + e + executable_extension for e in [
      "clang", "clang++", "clang-cpp", "clang-cl",
    ]
  ],
)

# ...

[peakschris]

This sounds good but I'm not sure how the templating would work, could you elaborate?

[rrbutani]

I'm not very familiar with Windows but IIUC the binaries require the DLLs in bin/ in the Windows release tarball to function; I think we need to add them to bin and clang, as, ar, etc?

[peakschris]

I don't need the DLLs as I already have them installed via MSVC on our systems. But for others without MSVC this is a good point. Have a look at my fix, as I don't know how these filegroups will be consumed downstream I'm not sure if the DLLs should go in srcs or data or simply allow downstream to consume separately from the dlls filegroup.

[peakschris]

If you haven't come across it already, #154 may be of interest to you; it uses cc_toolchain_config from @bazel_tools//tools/cpp:windows_cc_toolchain_config.bzl which we'll probably also want to eventually do.

This is a great reference, thank you! I don't understand cross compilation, but could give a go at integrating the windows_cc_toolchain_config to a level where it works on a windows machine. I'd like to do this as a followup PR if that's OK. Maybe someone else could take on the cross compilation aspect, I can see that would be hugely valuable for CI runners.