CI: Add runner that builds MSVC target with clang/clang++

[mmuetzel]

This is just to demonstrate the issue discussed in #750.

The issue probably needs a fix in upstream CMake (not in SuiteSparse). Please, do not merge (until this is fixed upstream and the fix made its way to Visual Studio).

[mmuetzel]

The (upstream) issue is that the OpenMP libraries aren't set for some reason by FindOpenMP.cmake:

-- OpenMP C libraries:       
-- OpenMP C include:         
-- OpenMP C flags:          -fopenmp=libomp 

IIUC, they should be set to the same as when building with clang-cl:

-- OpenMP C libraries:      C:/Program Files/Microsoft Visual Studio/2022/Enterprise/VC/Tools/MSVC/14.37.32822/lib/x64/libomp.lib 
-- OpenMP C include:         
-- OpenMP C flags:          -Xclang -fopenmp 

[mmuetzel]

The following change to the file provided by CMake fixes the issue for me locally:

diff --git a/Modules/FindOpenMP.cmake b/Modules/FindOpenMP.cmake
index 69099f7620..a311b915dd 100644
--- a/Modules/FindOpenMP.cmake
+++ b/Modules/FindOpenMP.cmake
@@ -224,7 +224,8 @@ function(_OPENMP_GET_FLAGS LANG FLAG_MODE OPENMP_FLAG_VAR OPENMP_LIB_NAMES_VAR)
       OUTPUT_VARIABLE OpenMP_TRY_COMPILE_OUTPUT
     )
 
-    if(OpenMP_COMPILE_RESULT_${FLAG_MODE}_${OPENMP_PLAIN_FLAG})
+    if(OpenMP_COMPILE_RESULT_${FLAG_MODE}_${OPENMP_PLAIN_FLAG} AND
+       NOT CMAKE_${LANG}_SIMULATE_ID STREQUAL "MSVC")
       set("${OPENMP_FLAG_VAR}" "${OPENMP_FLAG}" PARENT_SCOPE)
 
       if(CMAKE_${LANG}_VERBOSE_FLAG)

I don't know if this is the correct change though or if this might break something else.

[mmuetzel]

#761 would probably fix the build error where it is failing currently.

[mmuetzel]

With the last experiment, the OpenMP libraries are detected correctly also for the runner that uses clang targeting MSVC:
https://github.com/DrTimothyAldenDavis/SuiteSparse/actions/runs/7804679990/job/21287194730#step:15:152

-- OpenMP C libraries:      C:/Program Files/Microsoft Visual Studio/2022/Enterprise/VC/Tools/MSVC/14.37.32822/lib/x64/libomp.lib 
-- OpenMP C include:         
-- OpenMP C flags:          -fopenmp=libomp