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/6/llvm.nix

@@ -21,7 +21,7 @@ let
 
   # Used when creating a versioned symlinks of libLLVM.dylib
   versionSuffixes = with stdenv.lib;
-    let parts = splitString "." release_version; in
+    let parts = splitVersion release_version; in
     imap (i: _: concatStringsSep "." (take i parts)) parts;
 in