llvmPackages.libcxx: Include libc sources in input

Without this, after llvm 6c4267fb1779bc5550bb413f33250f9365acfbc6  you
get a build failure because libcxx can't find headers which now live in
the libc tree.

Ref: https://discourse.llvm.org/t/rfc-project-hand-in-hand-llvm-libc-libc-code-sharing/77701/18
Ref: https://github.com/llvm/llvm-project/commit/6c4267fb1779bc5550bb413f33250f9365acfbc6
Signed-off-by: Peter Waller <p@pwaller.net>

pkgs/development/compilers/llvm/common/libcxx/default.nix

@@ -44,6 +44,8 @@ let
       cp -r ${monorepoSrc}/llvm/utils "$out/llvm"
     '' + (lib.optionalString (lib.versionAtLeast release_version "14") ''
       cp -r ${monorepoSrc}/third-party "$out"
+    '') + (lib.optionalString (lib.versionAtLeast release_version "20") ''
+      cp -r ${monorepoSrc}/libc "$out"
     '') + ''
       cp -r ${monorepoSrc}/runtimes "$out"
     '' + (lib.optionalString (cxxabi == null) ''