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
Signed-off-by: william.r.dieter <william.r.dieter@intel.com>
37 jobs for !5795 with flag-fix in 6 minutes and 29 seconds (queued for 2 seconds)
latest detached
Status Name Job ID Coverage
  Build
manual build:centos6-x86_64 #5528035
cmake build docker linux manual
passed build:debian10-iwyu #5528032
cmake build docker linux

00:01:55

manual build:fedora33-ninja #5528036
cmake build docker linux linux-3.17 manual
passed build:fedora33-sphinx #5528034
cmake build docker linux linux-3.17

00:06:29

passed build:fedora33-tidy #5528033
cmake build docker linux linux-3.17

00:04:01

manual build:macos-makefiles #5528038
cmake macos shell xcode-12.3 nonconcurrent manual
manual build:macos-ninja #5528037
cmake macos shell xcode-12.3 nonconcurrent manual
manual build:windows-vs2019-x64-ninja #5528039
cmake windows shell vs2019 msvc-19.28 nonconcurrent manual
 
  Test
created test:fedora33-ninja #5528040
cmake docker linux linux-3.17 x11
created test:macos-makefiles #5528042
nonconcurrent cmake macos shell xcode-12.3
created test:macos-ninja #5528041
cmake macos shell xcode-12.3 nonconcurrent
created test:windows-vs2019-x64-ninja #5528043
cmake windows shell vs2019 msvc-19.28 nonconcurrent
 
  Test Ext
created test:cuda10.2-nvidia #5528046
cmake cuda-rt docker linux
created test:debian10-ninja #5528044
cmake build docker linux linux-3.17
created test:fedora33-makefiles #5528045
cmake build docker linux linux-3.17
created test:fedora33-ninja-multi #5528047
cmake build docker linux linux-3.17
created test:macos-xcode #5528048
cmake macos shell xcode-12.3 concurrent
created test:windows-vs2019-x64 #5528049
cmake windows shell vs2019 msvc-19.28 concurrent
 
  External
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