IntelLLVM: Fix C/C++ standard level flags on Windows

In commit a90d2a9e (IntelLLVM: Add support for Intel LLVM-based
compilers, 2020-11-02, v3.20.0-rc1~89^2~20) we accidentally left out
activation of the C/C++ standard level selection logic when IntelLLVM is
targeting the MSVC ABI.

Fixes: #22388