fix clangd wrapper

* get libc-cflags and libcxx-cxxflags from the sibling clang
  derivation.

* fix the order of those for C++ (libc-cflags use -idirafter for a
  reason).

pkgs/development/tools/clang-tools/wrapper

@@ -1,20 +1,27 @@
 #!/bin/sh
 
 buildcpath() {
-  local path
+  local path after
   while (( $# )); do
     case $1 in
         -isystem)
             shift
             path=$path${path:+':'}$1
+            ;;
+        -idirafter)
+            shift
+            after=$after${after:+':'}$1
+            ;;
     esac
     shift
   done
-  echo $path
+  echo $path${after:+':'}$after
 }
 
-export CPATH=${CPATH}${CPATH:+':'}$(buildcpath ${NIX_CFLAGS_COMPILE})
-export CPATH=${CPATH}${CPATH:+':'}@libc_includes@
-export CPLUS_INCLUDE_PATH=${CPATH}${CPATH:+':'}@libcpp_includes@
+export CPATH=${CPATH}${CPATH:+':'}$(buildcpath ${NIX_CFLAGS_COMPILE} \
+                                               $(<@clang@/nix-support/libc-cflags))
+export CPLUS_INCLUDE_PATH=${CPLUS_INCLUDE_PATH}${CPLUS_INCLUDE_PATH:+':'}$(buildcpath ${NIX_CFLAGS_COMPILE} \
+                                                                                      $(<@clang@/nix-support/libcxx-cxxflags) \
+                                                                                      $(<@clang@/nix-support/libc-cflags))
 
-exec -a "$0" @clang@/bin/$(basename $0) "$@"
+exec -a "$0" @unwrapped@/bin/$(basename $0) "$@"