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