llvmPackages: Fix more inconsistencies

The main thing was using `llvm_meta` in all versions.

Secondarily:

 - libunwindx7: Forgot to split outputs

 - libcxx{,abi} 12: Forgot to apply output-splitting patches.

 - simplify `useLLVM` stdenv-switching logic.

 - openmp always gets its own directory

pkgs/development/compilers/llvm/8/openmp/default.nix

@@ -0,0 +1,34 @@
+{ lib
+, stdenv
+, llvm_meta
+, fetch
+, cmake
+, llvm
+, perl
+, version
+}:
+
+stdenv.mkDerivation {
+  pname = "openmp";
+  inherit version;
+
+  src = fetch "openmp" "0b3jlxhqbpyd1nqkpxjfggm5d9va5qpyf7d4i5y7n4a1mlydv19y";
+
+  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 ];
+  };
+}