tree-wide: replace uses of splitString "." with lib.versions

Quoting from the splitString docstring:

   NOTE: this function is not performant and should never be used.

This replaces trivial uses of splitString for splitting version
strings with the (potentially builtin) splitVersion.

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

@@ -25,7 +25,7 @@ let
 
   # Used when creating a version-suffixed symlink of libLLVM.dylib
   shortVersion = with stdenv.lib;
-    concatStringsSep "." (take 1 (splitString "." release_version));
+    concatStringsSep "." (take 1 (splitVersion release_version));
 
 in stdenv.mkDerivation ({
   name = "llvm-${version}";