llvmPackages_git: Copy from llvmPackages_12

The purpose of this package is to continuously improve the LLVM
packaging in Nixpkgs without causing a lot of rebuilds and provide more
recent LLVM builds for users. For more details see:
https://github.com/NixOS/nixpkgs/issues/114828

pkgs/development/compilers/llvm/git/openmp/default.nix

@@ -0,0 +1,34 @@
+{ lib
+, stdenv
+, llvm_meta
+, fetch
+, cmake
+, llvm
+, perl
+, version
+}:
+
+stdenv.mkDerivation rec {
+  pname = "openmp";
+  inherit version;
+
+  src = fetch pname "0z8n1wanby6aq3i7d91mgk72hb33zfl5blayk0a22cs7l8i706zb";
+
+  nativeBuildInputs = [ cmake perl ];
+  buildInputs = [ llvm ];
+
+  meta = llvm_meta // {
+    homepage = "https://openmp.llvm.org/";
+    description = "Support for the OpenMP language";
+    longDescription = ''
+      The OpenMP subproject of LLVM contains the components required to build an
+      executable OpenMP program that are outside the compiler itself.
+      Contains the code for the runtime library against which code compiled by
+      "clang -fopenmp" must be linked before it can run and the library that
+      supports offload to target devices.
+    '';
+    # "All of the code is dual licensed under the MIT license and the UIUC
+    # License (a BSD-like license)":
+    license = with lib.licenses; [ mit ncsa ];
+  };
+}