FindMPI: Fix finding IntelLLVM wrappers on Windows
In !9170 (merged) we added mpiicpx.bat
for C++ on Windows, but that is
a GNU-like front-end that we do not yet support. Use mpiicx.bat
for
both C and C++ on Windows, just as we use icx
to compile both.
Fixes: #25807 (closed)
Backport: release