llvmPackages: keep darwin at llvm5

My previous attempts to bump the Darwin stdenv version didn't
seem to work, but AFAIK doing so should be straightforward
for anyone with a Darwin builder and the interest in tackling this.

While not the preferred solution, adding a conditional lets us move on,
as we missed 6 entirely due to no one getting around to resolving the
Darwin bootstrap accordingly.

pkgs/top-level/all-packages.nix

@@ -7544,7 +7544,7 @@ in
   llvm_37 = llvmPackages_37.llvm;
   llvm_35 = llvmPackages_35.llvm;
 
-  llvmPackages = recurseIntoAttrs llvmPackages_7;
+  llvmPackages = recurseIntoAttrs (if stdenv.isDarwin then llvmPackages_5 else llvmPackages_7);
 
   llvmPackages_35 = callPackage ../development/compilers/llvm/3.5 ({
     isl = isl_0_14;