Windows builds with clang-cl.exe

[htuch]

In order to reduce the need to engineer workarounds when interoperating with projects such as Chromium that build with clang-cl, and to provide greater consistency with the rest of our Clang-based builds, we should use clang-cl.exe instead of cl.exe. Both are compatible with MSVC.

The main blocker here appears to be google/googletest#2490.

[htuch]

@wrowe I've been told that the gmock issue may no longer apply (see https://chromium-review.googlesource.com/c/chromium/src/+/2310189). Unclear if this works in Clang 10 or requires Clang 11.

[sunjayBhatia]

See envoyproxy/envoy-build-tools#86 for installing clang in the Windows build container

[sunjayBhatia]

Current issues we faced building locally:

• curl does not build as expected, extra arguments are passed to the llvm link tool that are not relevant for libraries, this was happening with fastbuild, have not yet tried with opt, we've seen major differences in foreign cc components with different build modes (CMake projects treatment of debug builds is often faulty) so that could be something to explore
• clang is requiring all mocks to have the override specifier, which may ultimately be correct, but clang on non-Windows does not warn/error on this
  • as a general point, we need to figure out the correct set of flags to pass to clang-cl to be consistent with non-Windows clang
• We still need to figure out the flags/options to set in the clang-cl rbe toolchain

[sunjayBhatia]

See envoyproxy/envoy-build-tools#88 for additions and issues with the clang-cl toolchain config

[sunjayBhatia]

See also bazelbuild/bazel-toolchains#901

[sunjayBhatia]

Still blocked on envoyproxy/envoy-build-tools#88 getting merged, cc @lizan when you get a chance

[sunjayBhatia]

See #12776 for clang-cl RBE toolchain

[wrowe]

We are currently working on a fork locally to determine the major roadblocks, while using the newly created toolchain and docker image. Expecting to create a PR shortly to demonstrate remaining breakage and move this forward.