llvm-3.8: build shared library on darwin (#17671)

- Enable the shared library build on darwin by default to match other
  platforms.

- Fix the dylib file's name in the store

- Symlink a versioned name as some tooling expects this.

pkgs/development/compilers/llvm/3.8/llvm.nix

@@ -14,7 +14,7 @@
 , compiler-rt_src
 , libcxxabi
 , debugVersion ? false
-, enableSharedLibraries ? !stdenv.isDarwin
+, enableSharedLibraries ? true
 }:
 
 let
@@ -69,6 +69,11 @@ in stdenv.mkDerivation rec {
     paxmark m bin/{lli,llvm-rtdyld}
   '';
 
+  postInstall = stdenv.lib.optionalString (stdenv.isDarwin && enableSharedLibraries) ''
+    install_name_tool -id $out/lib/libLLVM.dylib $out/lib/libLLVM.dylib
+    ln -s $out/lib/libLLVM.dylib $out/lib/libLLVM-${version}.dylib
+  '';
+
   enableParallelBuilding = true;
 
   passthru.src = src;