IntelLLVM: Remove unsupported -imsvc system include flag

Clang-cl supports the -imsvc flag to tell the compiler an include directory is intended for system paths. icx does not accept this flag, even on MSVC platforms, so do not tell CMake that it exists.

Fixes: #21801 (closed)
Backport: release
Topic-rename: IntelLLVM-no-imsvc