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/11/lldb/default.nix

@@ -1,4 +1,4 @@
-{ lib, stdenv
+{ lib, stdenv, llvm_meta
 , fetch
 , cmake
 , zlib
@@ -76,11 +76,15 @@ stdenv.mkDerivation (rec {
     ln -s $out/bin/llvm-vscode $out/share/vscode/extensions/llvm-org.lldb-vscode-0.1.0/bin
   '';
 
-  meta = with lib; {
+  meta = llvm_meta // {
+    homepage = "https://lldb.llvm.org/";
     description = "A next-generation high-performance debugger";
-    homepage    = "https://lldb.llvm.org";
-    license     = licenses.ncsa;
-    platforms   = platforms.all;
+    longDescription = ''
+      LLDB is a next generation, high-performance debugger. It is built as a set
+      of reusable components which highly leverage existing libraries in the
+      larger LLVM Project, such as the Clang expression parser and LLVM
+      disassembler.
+    '';
   };
 } // lib.optionalAttrs enableManpages {
   pname = "lldb-manpages";
@@ -104,5 +108,7 @@ stdenv.mkDerivation (rec {
 
   doCheck = false;
 
-  meta.description = "man pages for LLDB ${version}";
+  meta = llvm_meta // {
+    description = "man pages for LLDB ${version}";
+  };
 })