llvm_*,clang_*: allow src overrides

pkgs/development/compilers/llvm/8/clang/default.nix

@@ -8,8 +8,10 @@ let
   self = stdenv.mkDerivation ({
     name = "clang-${version}";
 
+    src = fetch "cfe" "0svk1f70hvpwrjp6x5i9kqwrqwxnmcrw5s7f4cxyd100mdd12k08";
+
     unpackPhase = ''
-      unpackFile ${fetch "cfe" "0svk1f70hvpwrjp6x5i9kqwrqwxnmcrw5s7f4cxyd100mdd12k08"}
+      unpackFile $src
       mv cfe-${version}* clang
       sourceRoot=$PWD/clang
       unpackFile ${clang-tools-extra_src}

pkgs/development/compilers/llvm/8/llvm.nix

@@ -23,9 +23,6 @@
 let
   inherit (stdenv.lib) optional optionals optionalString;
 
-  src = fetch "llvm" "0k124sxkfhfi1rca6kzkdraf4axhx99x3cw2rk55056628dvwwl8";
-  polly_src = fetch "polly" "1x4xv3j226rqdddp7b61d71wsx2b8vmmri02ycx27y2fg7ba7xg3";
-
   # Used when creating a version-suffixed symlink of libLLVM.dylib
   shortVersion = with stdenv.lib;
     concatStringsSep "." (take 1 (splitString "." release_version));
@@ -33,12 +30,15 @@ let
 in stdenv.mkDerivation (rec {
   name = "llvm-${version}";
 
+  src = fetch "llvm" "0k124sxkfhfi1rca6kzkdraf4axhx99x3cw2rk55056628dvwwl8";
+  polly_src = fetch "polly" "1x4xv3j226rqdddp7b61d71wsx2b8vmmri02ycx27y2fg7ba7xg3";
+
   unpackPhase = ''
-    unpackFile ${src}
+    unpackFile $src
     mv llvm-${version}* llvm
     sourceRoot=$PWD/llvm
   '' + optionalString enablePolly ''
-    unpackFile ${polly_src}
+    unpackFile $polly_src
     mv polly-* $sourceRoot/tools/polly
   '';
 
@@ -142,8 +142,6 @@ in stdenv.mkDerivation (rec {
 
   enableParallelBuilding = true;
 
-  passthru.src = src;
-
   meta = {
     description = "Collection of modular and reusable compiler and toolchain technologies";
     homepage    = http://llvm.org/;