Wrap the clang produced during the build

--- a/tools/driver/CMakeLists.txt
+++ b/tools/driver/CMakeLists.txt
@@ -59,6 +59,13 @@ endif()
 
 add_dependencies(clang clang-resource-headers)
 
+# Nix: wrap the clang build.
+add_custom_command(
+  TARGET clang POST_BUILD
+  COMMAND nix-swift-make-clang-wrapper $<TARGET_FILE:clang>
+  VERBATIM
+)
+
 if(NOT CLANG_LINKS_TO_CREATE)
   set(CLANG_LINKS_TO_CREATE clang++ clang-cl clang-cpp)
 endif()