llvmPackages: add asl20-llvm to licenses for v19+

As per llvm/llvm-project#92394 contributions made after June 1st, 2024 are
licensed exclusively under `Apache-2.0 WITH LLVM-exception` rather than dual
licensed.

As per this post, this change is timed so that it only affects version 19 and
newer: https://discourse.llvm.org/t/relicensing-next-step-dropping-requirement-to-contribute-also-under-the-legacy-license/78351

pkgs/development/compilers/llvm/common/common-let.nix

@@ -8,7 +8,10 @@
 
 rec {
   llvm_meta = {
-    license = lib.licenses.ncsa;
+    license = with lib.licenses; [ ncsa ] ++
+      # Contributions after June 1st, 2024 are only licensed under asl20-llvm:
+      # https://github.com/llvm/llvm-project/pull/92394
+      lib.optional (lib.versionAtLeast release_version "19") asl20-llvm;
     maintainers = lib.teams.llvm.members;
 
     # See llvm/cmake/config-ix.cmake.