[android] Escape include paths in Android.

Before the result of `_add_variant_c_compile_flags` was a string, so appending several "-isystem" was not a problem. With #29451 the rules have changed since the list is now handled by CMake, and it deduplicates the repeated members in the list. Thanks, CMake.

Should fix the Android CI builds that were failing since the merging of #29451.

Author: drodriguez

cmake/modules/AddSwift.cmake

@@ -363,7 +363,7 @@ function(_add_variant_c_compile_flags)
     swift_android_libcxx_include_paths(CFLAGS_CXX_INCLUDES)
     swift_android_include_for_arch("${CFLAGS_ARCH}" "${CFLAGS_ARCH}_INCLUDE")
     foreach(path IN LISTS CFLAGS_CXX_INCLUDES ${CFLAGS_ARCH}_INCLUDE)
-      list(APPEND result -isystem;${path})
+      list(APPEND result "SHELL:${CMAKE_INCLUDE_SYSTEM_FLAG_C}${path}")
     endforeach()
     list(APPEND result "-D__ANDROID_API__=${SWIFT_ANDROID_API_LEVEL}")
   elseif(CFLAGS_SDK STREQUAL WINDOWS)