llvm-{4,5}: don't build entire project to get the manpages

Much cheaper to build this way.

Also this gives them a different name to avoid confusion
and clutter when using `nix-env -qaP` or similar.

pkgs/development/compilers/llvm/5/default.nix

@@ -20,7 +20,7 @@ let
   # Add man output without introducing extra dependencies.
   overrideManOutput = drv:
     let drv-manpages = drv.override { enableManpages = true; }; in
-    drv // { man = drv-manpages.man; /*outputs = drv.outputs ++ ["man"];*/ };
+    drv // { man = drv-manpages.out; /*outputs = drv.outputs ++ ["man"];*/ };
 
   llvm = callPackage ./llvm.nix {
     inherit compiler-rt_src stdenv;