[ImportVerilog] Disable Slang frontend in MSVC CI builds

MSVC fails to build the Slang 3 dependency due to issues in Slang's
`MathUtils.h`. (The file is missing an `#include <limits>`.) Since we
are planning to upgrade to Slang 4 or 5 anyway, don't bother setting up
a full workaround for this issue yet and instead just disable Slang in
MSVC CI builds and installs. Once the Verilog frontend integration
matures and we update Slang, we can go back in and enable MSVC builds
again.

.github/workflows/unifiedBuildTestAndInstall.yml

@@ -177,7 +177,7 @@ jobs:
           ${{ steps.setup-windows.outputs.setup }}
           mkdir build
           cd build
-          cmake -G Ninja -S "$(pwd)/../llvm/llvm" $EXTRA_CMAKE_ARGS -DCMAKE_BUILD_TYPE=${{ inputs.cmake_build_type }} -DBUILD_SHARED_LIBS=${{ inputs.build_shared_libs }} -DLLVM_BUILD_TOOLS=ON -DLLVM_BUILD_EXAMPLES=OFF -DLLVM_ENABLE_ASSERTIONS=${{ inputs.llvm_enable_assertions }} -DLLVM_ENABLE_PROJECTS=mlir -DLLVM_EXTERNAL_PROJECTS=circt -DLLVM_EXTERNAL_CIRCT_SOURCE_DIR=".." -DLLVM_STATIC_LINK_CXX_STDLIB=ON -DLLVM_ENABLE_TERMINFO=OFF -DLLVM_PARALLEL_LINK_JOBS=1 -DLLVM_TARGETS_TO_BUILD="host" -DLLVM_FORCE_ENABLE_STATS=${{ inputs.llvm_force_enable_stats }} -DLLVM_ENABLE_ZSTD=OFF -DVERILATOR_DISABLE=ON -DLLVM_PARALLEL_LINK_JOBS=1 -DCIRCT_RELEASE_TAG_ENABLED=ON -DCIRCT_RELEASE_TAG=firtool -DCMAKE_EXPORT_COMPILE_COMMANDS=OFF -DCMAKE_C_COMPILER=${{ inputs.cmake_c_compiler }} -DCMAKE_CXX_COMPILER=${{ inputs.cmake_cxx_compiler }} ${{ steps.configure-sccache.outputs.enable_sccache }} -DCMAKE_INSTALL_PREFIX="$(pwd)/../install" -DLLVM_INSTALL_UTILS=ON -DCIRCT_SLANG_FRONTEND_ENABLED=ON
+          cmake -G Ninja -S "$(pwd)/../llvm/llvm" $EXTRA_CMAKE_ARGS -DCMAKE_BUILD_TYPE=${{ inputs.cmake_build_type }} -DBUILD_SHARED_LIBS=${{ inputs.build_shared_libs }} -DLLVM_BUILD_TOOLS=ON -DLLVM_BUILD_EXAMPLES=OFF -DLLVM_ENABLE_ASSERTIONS=${{ inputs.llvm_enable_assertions }} -DLLVM_ENABLE_PROJECTS=mlir -DLLVM_EXTERNAL_PROJECTS=circt -DLLVM_EXTERNAL_CIRCT_SOURCE_DIR=".." -DLLVM_STATIC_LINK_CXX_STDLIB=ON -DLLVM_ENABLE_TERMINFO=OFF -DLLVM_PARALLEL_LINK_JOBS=1 -DLLVM_TARGETS_TO_BUILD="host" -DLLVM_FORCE_ENABLE_STATS=${{ inputs.llvm_force_enable_stats }} -DLLVM_ENABLE_ZSTD=OFF -DVERILATOR_DISABLE=ON -DLLVM_PARALLEL_LINK_JOBS=1 -DCIRCT_RELEASE_TAG_ENABLED=ON -DCIRCT_RELEASE_TAG=firtool -DCMAKE_EXPORT_COMPILE_COMMANDS=OFF -DCMAKE_C_COMPILER=${{ inputs.cmake_c_compiler }} -DCMAKE_CXX_COMPILER=${{ inputs.cmake_cxx_compiler }} ${{ steps.configure-sccache.outputs.enable_sccache }} -DCMAKE_INSTALL_PREFIX="$(pwd)/../install" -DLLVM_INSTALL_UTILS=ON
 # Optionally test
       - name: Test CIRCT
         if: inputs.runTests