llvmPackages: unify version for all platforms but Darwin (#343245)

  libllvm = llvmPackages.libllvm;

  llvm-manpages = llvmPackages.llvm-manpages;

  # Please remove all this logic when bumping to LLVM 19 and make this

  # a simple alias.

  llvmPackages = let

    # This returns the minimum supported version for the platform. The

    # assumption is that or any later version is good.

    choose = platform:

      /**/ if platform.isDarwin then 16

      else if platform.isFreeBSD then 18

      else if platform.isOpenBSD then 18

      else if platform.isAndroid then 12

      else if platform.isLinux then 18

      else if platform.isWasm then 16

      # For unknown systems, assume the latest version is required.

      else 18;

    choose = platform: if platform.isDarwin then 16 else 18;

    # We take the "max of the mins". Why? Since those are lower bounds of the

    # supported version set, this is like intersecting those sets and then

    # taking the min bound of that.