Clang: Use -imsvc for system include only with MSVC-like front-end
In !5231 (merged) 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 (closed)
Backport: release