IntelLLVM: Avoid finding not-yet-supported icpx on Windows

Intel oneAPI 2023.0 added the `icpx` compiler front-end on Windows.
It uses a GNU-like command-line, and is not yet supported by CMake.
Avoid finding `icpx` as the CXX compiler on Windows until support
is added.

Fixes: #24266
Issue: #24314
1 file changed