xcode: add 9.4 sdk

This is the newest sdk. I have skipped 9.3 for now but someone can
certainly add it if they need it for some reason.

Also I added a generic "xcode" that will always point to the newest
xcode that is available in Nixpkgs.

pkgs/os-specific/darwin/xcode/default.nix

···
 
   in app.overrideAttrs ( oldAttrs: oldAttrs // { inherit meta; });
 
-in {
+in rec {
   xcode_8_1 = requireXcode "8.1" "18xjvfipwzia66gm3r9p770xdd4r375vak7chw5vgqnv9yyjiq2n";
   xcode_8_2 = requireXcode "8.2" "13nd1zsfqcp9hwp15hndr0rsbb8rgprrz7zr2ablj4697qca06m2";
   xcode_9_1 = requireXcode "9.1" "0ab1403wy84ys3yn26fj78cazhpnslmh3nzzp1wxib3mr1afjvic";
   xcode_9_2 = requireXcode "9.2" "1bgfgdp266cbbqf2axcflz92frzvhi0qw0jdkcw6r85kdpc8dj4c";
+  xcode_9_4 = requireXcode "9.4" "6731381785075602a52489f7ea47ece8f6daf225007ba3ffae1fd59b1c0b5f01";
+  xcode = xcode_9_4;
 }

pkgs/top-level/darwin-packages.nix

···
 
   usr-include = callPackage ../os-specific/darwin/usr-include { };
 
-  inherit (callPackages ../os-specific/darwin/xcode { } ) xcode_8_1 xcode_8_2 xcode_9_1 xcode_9_2;
+  inherit (callPackages ../os-specific/darwin/xcode { } )
+          xcode_8_1 xcode_8_2 xcode_9_1 xcode_9_2 xcode_9_4 xcode;
 
   CoreSymbolication = callPackage ../os-specific/darwin/CoreSymbolication { };