rustPlatform: fix darwin build failure

[zowoq]

Motivation for this change

Fix https://hydra.nixos.org/build/128509615/nixlog/2

Kind of reverts the clang changes in b837bd4.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

[marsam]

I think we could apply it conditionally on all-packages.nix

diff --git i/pkgs/top-level/all-packages.nix w/pkgs/top-level/all-packages.nix
index c74f2249fc8..c5bfc9416f8 100644
--- i/pkgs/top-level/all-packages.nix
+++ w/pkgs/top-level/all-packages.nix
@@ -9661,11 +9661,11 @@ in
   # So this commit doesn't remove the 1.45.2 release.
   rust_1_45 = callPackage ../development/compilers/rust/1_45.nix {
     inherit (darwin.apple_sdk.frameworks) CoreFoundation Security;
-    llvmPackages = llvmPackages_10;
+    llvmPackages = if stdenv.cc.isClang then llvmPackages_5 else llvmPackages_10;
   };
   rust_1_46 = callPackage ../development/compilers/rust/1_46.nix {
     inherit (darwin.apple_sdk.frameworks) CoreFoundation Security;
-    llvmPackages = llvmPackages_10;
+    llvmPackages = if stdenv.cc.isClang then llvmPackages_5 else llvmPackages_10;
   };
   rust = rust_1_46;

[zowoq]

Merging as it's blocking nixpkgs-unstable.