IntelLLVM: support marking include paths as SYSTEM directories

Ben Boeckelrequested to merge
ben.boeckel/cmake:IntelLLVM-isystem-flag into master

See: https://discourse.cmake.org/t/icx-on-windows-supports-external-i/8739

I've not been able to find documentation for this. I've sent out queries about it though.

Edited by Ben Boeckel

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