rustc: update llvm on darwin to current

We downgraded when LLVM 10 wasn't working well on Darwin: NixOS#101136.
Now that we're using LLVM 11, the issue is no longer present.

(cherry picked from commit df2cdfa)

pkgs/development/compilers/rust/1_50.nix

@@ -15,7 +15,7 @@
 , CoreFoundation, Security
 , pkgsBuildTarget, pkgsBuildBuild, pkgsBuildHost
 , makeRustPlatform
-, llvmPackages_5, llvm_11
+, llvmPackages_11, llvm_11
 } @ args:
 
 import ./default.nix {
@@ -26,7 +26,7 @@ import ./default.nix {
   llvmSharedForHost = pkgsBuildHost.llvm_11.override { enableSharedLibraries = true; };
   llvmSharedForTarget = pkgsBuildTarget.llvm_11.override { enableSharedLibraries = true; };
 
-  llvmBootstrapForDarwin = llvmPackages_5;
+  llvmBootstrapForDarwin = llvmPackages_11;
 
   # For use at runtime
   llvmShared = llvm_11.override { enableSharedLibraries = true; };
@@ -55,4 +55,4 @@ import ./default.nix {
   ];
 }
 
-(builtins.removeAttrs args [ "fetchpatch" "pkgsBuildHost" "llvmPackages_5" "llvm_11"])
+(builtins.removeAttrs args [ "fetchpatch" "pkgsBuildHost" "llvmPackages_11" "llvm_11"])