[CI] Update Windows premerge testing to use clang-cl.exe

Now that the Windows container contains clang, use it for building
the premerge tests. Measurements show this is significantly faster
than using msvc cl. Note we had to disable two warnings
-Wc++98-compat and -Wc++14-compat, to make this work with 'check-mlir'
on Windows (clang generates a lot of warnings that msvc cl does not).

Author: cmtice

.ci/monolithic-windows.sh

@@ -23,8 +23,8 @@ runtimes_targets="${4}"
 start-group "CMake"
 pip install -q -r "${MONOREPO_ROOT}"/.ci/all_requirements.txt
 
-export CC=cl
-export CXX=cl
+export CC=C:\clang\clang-msvc\bin\clang-cl.exe
+export CXX=C:\clang\clang-msvc\bin\clang-cl.exe
 export LD=link
 
 # The CMAKE_*_LINKER_FLAGS to disable the manifest come from research
@@ -49,10 +49,12 @@ cmake -S "${MONOREPO_ROOT}"/llvm -B "${BUILD_DIR}" \
       -D CMAKE_EXE_LINKER_FLAGS="/MANIFEST:NO" \
       -D CMAKE_MODULE_LINKER_FLAGS="/MANIFEST:NO" \
       -D CMAKE_SHARED_LINKER_FLAGS="/MANIFEST:NO" \
+      -D CMAKE_CXX_FLAGS="-Wno-c++98-compat -Wno-c++14-compat" \
       -D LLVM_ENABLE_RUNTIMES="${runtimes}"
 
 start-group "ninja"
 
+
 # Targets are not escaped as they are passed as separate arguments.
 ninja -C "${BUILD_DIR}" -k 0 ${targets} |& tee ninja.log
 cp ${BUILD_DIR}/.ninja_log ninja.ninja_log