llvm4.0: Rename "4.0" (and _40) to "4" (and _4).

This reflects upstream versioning change, and allows
us to replace 4.0 with 4.1 (which is now a minor revision)
without changing the attribute name.

Thanks to @vcunat for the idea.

pkgs/development/compilers/llvm/4/default.nix

@@ -8,7 +8,7 @@ let
 
   fetch = name: sha256: fetchurl {
     url = "http://llvm.org/pre-releases/${release_version}/${rc}/${name}-${version}.src.tar.xz";
-    # Once 4.0 is released, use this instead:
+    # Once 4 is released, use this instead:
     # url = "http://llvm.org/releases/${release-version}/${name}-${version}.src.tar.xz";
     inherit sha256;
   };

pkgs/top-level/all-packages.nix

@@ -4759,7 +4759,7 @@ with pkgs;
 
   clang = llvmPackages.clang;
 
-  clang_40 = lowPrio llvmPackages_40.clang;
+  clang_4 = lowPrio llvmPackages_4.clang;
   clang_39 = llvmPackages_39.clang;
   clang_38 = llvmPackages_38.clang;
   clang_37 = llvmPackages_37.clang;
@@ -5314,13 +5314,13 @@ with pkgs;
 
   lizardfs = callPackage ../tools/filesystems/lizardfs { };
 
-  lld = lowPrio llvmPackages_40.lld;
+  lld = lowPrio llvmPackages_4.lld;
 
   lldb = llvmPackages.lldb;
 
   llvm = llvmPackages.llvm;
 
-  llvm_40 = lowPrio llvmPackages_40.llvm;
+  llvm_4 = lowPrio llvmPackages_4.llvm;
   llvm_39 = llvmPackages_39.llvm;
   llvm_38 = llvmPackages_38.llvm;
   llvm_37 = llvmPackages_37.llvm;
@@ -5358,7 +5358,7 @@ with pkgs;
     inherit (stdenvAdapters) overrideCC;
   };
 
-  llvmPackages_40 = callPackage ../development/compilers/llvm/4.0 {
+  llvmPackages_4 = callPackage ../development/compilers/llvm/4 {
     inherit (stdenvAdapters) overrideCC;
   };