emacs.pkgs.llvm-mode: replace trivialBuild with melpaBuild

Related: https://github.com/NixOS/nixpkgs/issues/278925

There are three major modes inside this package.  Ideally, we should
split it into three packages.  However, to keep backward
compatibility, it is not split.

pkgs/applications/editors/emacs/elisp-packages/manual-packages/llvm-mode/default.nix

@@ -1,14 +1,10 @@
-{ trivialBuild
-, llvmPackages
-}:
+{ melpaBuild, llvmPackages }:
 
-trivialBuild {
+melpaBuild {
   pname = "llvm-mode";
-  inherit (llvmPackages.llvm) src version;
+  inherit (llvmPackages.llvm) version;
 
-  postUnpack = ''
-    sourceRoot="$sourceRoot/llvm/utils/emacs"
-  '';
+  src = "${llvmPackages.llvm.src}/llvm/utils/emacs";
 
   meta = {
     inherit (llvmPackages.llvm.meta) homepage license;