[CI] Update Windows premerge testing to use clang-cl.exe (llvm#164900)

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 four warnings -Wc++98-compat,
-Wc++14-compat,  -Wunsafe-buffer-usage, and -Wold-style-cast 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
@@ -49,6 +49,7 @@ 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 -Wno-unsafe-buffer-usage -Wno-old-style-cast" \
       -D LLVM_ENABLE_RUNTIMES="${runtimes}"
 
 start-group "ninja"