Clang: use -imsvc for system include dirs when running on Windows

Fixes #17808

Help/release/dev/clang-windows-system-includes.rst

@@ -0,0 +1,5 @@
+clang-windows-system-includes
+-----------------------------
+
+* The ``Clang`` compiler gained support for handling system include directories
+  when running on Windows.

Modules/Compiler/Clang.cmake

@@ -19,6 +19,7 @@ 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)