llvmPackages_12: reorganize files

Restructure pkgs/development/compilers/llvm/12 in a similar spirit
as #117433 to mirror the other llvm directories.

pkgs/development/compilers/llvm/12/libunwind/default.nix

@@ -0,0 +1,21 @@
+{ lib, stdenv, version, fetch, libcxx, llvm, cmake
+, enableShared ? !stdenv.hostPlatform.isStatic
+}:
+
+stdenv.mkDerivation rec {
+  pname = "libunwind";
+  inherit version;
+
+  src = fetch pname "0kaq75ygzv9dqfsx27pi5a0clipdjq6a9vghhb89d8k1rf20lslh";
+
+  postUnpack = ''
+    unpackFile ${libcxx.src}
+    mv libcxx-* libcxx
+    unpackFile ${llvm.src}
+    mv llvm-* llvm
+  '';
+
+  nativeBuildInputs = [ cmake ];
+
+  cmakeFlags = lib.optional (!enableShared) "-DLIBUNWIND_ENABLE_SHARED=OFF";
+}