all-packages: Define LLVM 11 as the default for Darwin

The Darwin stdenv takes some of the llvmPackages from the top-level pkgs
rather than explicitly from `llvmPackages_*`. This means you cannot
simply bump the LLVM version in the stdenv, you have to bump the default
version in all-packages.nix as well.

pkgs/top-level/all-packages.nix

···
     # 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 (if platform.isAarch64 then 11 else 7)
+      /**/ if platform.isDarwin then 11
       else if platform.isFreeBSD then 7
       else if platform.isAndroid then 12
       else if platform.isLinux then