Clang: Use -imsvc for system include only with MSVC-like front-end

In commit bb61c2d0 (Clang: use -imsvc for system include dirs when
running on Windows, 2020-09-16, v3.19.0-rc1~162^2) we added `-imsvc`
for all Clang compilers targeting the MSVC ABI.  However, the option
only exists for the MSVC-like front-end.  The GNU-like front-ends
use `-isystem`.

Fixes: #21789
36 jobs for !5792 with clang-imsvc in 7 minutes and 1 second (queued for 2 seconds)
detached
Status Job ID Name Coverage
  Build
manual #5525885
linux docker build cmake manual
build:centos6-release
passed #5525882
linux docker build cmake
build:debian10-iwyu

00:07:01

manual #5525886
linux-3.17 linux docker build cmake manual
build:fedora33-ninja
passed #5525884
linux-3.17 linux docker build cmake
build:fedora33-sphinx

00:05:35

passed #5525883
linux-3.17 linux docker build cmake
build:fedora33-tidy

00:02:39

manual #5525888
nonconcurrent xcode-12.3 shell macos cmake manual
build:macos-makefiles
manual #5525887
nonconcurrent xcode-12.3 shell macos cmake manual
build:macos-ninja
manual #5525889
nonconcurrent msvc-19.28 vs2019 shell windows cmake manual
build:windows-vs2019-x64-ninja
 
  Test
created #5525890
linux docker cmake x11 linux-3.17
test:fedora33-ninja
created #5525892
nonconcurrent xcode-12.3 shell macos cmake
test:macos-makefiles
created #5525891
nonconcurrent xcode-12.3 shell macos cmake
test:macos-ninja
created #5525893
nonconcurrent msvc-19.28 vs2019 shell windows cmake
test:windows-vs2019-x64-ninja
 
  Test Ext
created #5525896
linux docker cuda-rt cmake
test:cuda10.2-nvidia
created #5525894
linux-3.17 linux docker build cmake
test:debian10-ninja
created #5525895
linux-3.17 linux docker build cmake
test:fedora33-makefiles
created #5525897
linux-3.17 linux docker build cmake
test:fedora33-ninja-multi
created #5525898
concurrent xcode-12.3 shell macos cmake
test:macos-xcode
created #5525899
concurrent msvc-19.28 vs2019 shell windows cmake
test:windows-vs2019-x64
 
  External
passed cdash

passed ghostflow-check-master

passed ghostflow-check-release

passed ghostflow-stager

passed cdash

passed cdash

passed cdash

passed cdash

passed cdash

passed cdash

passed cdash

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager

passed ghostflow-stager