Merge pull request swiftlang#21374 from compnerd/batteries-included

build: add Windows includes to build

Author: compnerd

cmake/modules/AddSwift.cmake

@@ -307,6 +307,11 @@ function(_add_variant_c_compile_flags)
       list(APPEND result "\"${CMAKE_INCLUDE_FLAG_C}${path}\"")
     endforeach()
     list(APPEND result "-D__ANDROID_API__=${SWIFT_ANDROID_API_LEVEL}")
+  elseif(CFLAGS_SDK STREQUAL WINDOWS)
+    swift_windows_include_for_arch(${CFLAGS_ARCH} ${CFLAGS_ARCH}_INCLUDE)
+    foreach(path ${${CFLAGS_ARCH}_INCLUDE})
+      list(APPEND result "\"${CMAKE_INCLUDE_FLAG_C}${path}\"")
+    endforeach()
   endif()
 
   set("${CFLAGS_RESULT_VAR_NAME}" "${result}" PARENT_SCOPE)