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

In commit bb61c2d024 (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

Modules/Compiler/Clang.cmake

@@ -19,7 +19,6 @@ if("x${CMAKE_C_SIMULATE_ID}" STREQUAL "xMSVC"
     OR "x${CMAKE_CXX_SIMULATE_ID}" STREQUAL "xMSVC"
     OR "x${CMAKE_Fortran_SIMULATE_ID}" STREQUAL "xMSVC")
   macro(__compiler_clang lang)
-    set(CMAKE_INCLUDE_SYSTEM_FLAG_${lang} "-imsvc ")
   endmacro()
 else()
   include(Compiler/GNU)

Modules/Platform/Windows-Clang.cmake

@@ -169,6 +169,7 @@ if("x${CMAKE_C_SIMULATE_ID}" STREQUAL "xMSVC"
     macro(__windows_compiler_clang_base lang)
       set(_COMPILE_${lang} "${_COMPILE_${lang}_MSVC}")
       __windows_compiler_msvc(${lang})
+      set(CMAKE_INCLUDE_SYSTEM_FLAG_${lang} "-imsvc ")
     endmacro()
   else()
     cmake_policy(GET CMP0091 __WINDOWS_CLANG_CMP0091)