llvm_*,clang_*: allow src overrides

pkgs/development/compilers/llvm/5/clang/default.nix

@@ -8,8 +8,10 @@ let
   self = stdenv.mkDerivation ({
     name = "clang-${version}";
 
+    src = fetch "cfe" "0018520c4qxf5hgjdqgpz2dgl3faf4gsz87fdlb8zdmx99rfk77s";
+
     unpackPhase = ''
-      unpackFile ${fetch "cfe" "0018520c4qxf5hgjdqgpz2dgl3faf4gsz87fdlb8zdmx99rfk77s"}
+      unpackFile $src
       mv cfe-${version}* clang
       sourceRoot=$PWD/clang
       unpackFile ${clang-tools-extra_src}

pkgs/development/compilers/llvm/5/llvm.nix

@@ -15,8 +15,6 @@
 }:
 
 let
-  src = fetch "llvm" "0g1bbj2n6xv4p1n6hh17vj3vpvg56wacipc81dgwga9mg2lys8nm";
-
   # Used when creating a versioned symlinks of libLLVM.dylib
   versionSuffixes = with stdenv.lib;
     let parts = splitString "." release_version; in
@@ -26,8 +24,10 @@ in
 stdenv.mkDerivation (rec {
   name = "llvm-${version}";
 
+  src = fetch "llvm" "0g1bbj2n6xv4p1n6hh17vj3vpvg56wacipc81dgwga9mg2lys8nm";
+
   unpackPhase = ''
-    unpackFile ${src}
+    unpackFile $src
     mv llvm-${version}* llvm
     sourceRoot=$PWD/llvm
   '';
@@ -132,8 +132,6 @@ stdenv.mkDerivation (rec {
 
   enableParallelBuilding = true;
 
-  passthru.src = src;
-
   requiredSystemFeatures = [ "big-parallel" ];
   meta = {
     description = "Collection of modular and reusable compiler and toolchain technologies";