llvmPackages*.libcxx*: Always use the attribute name for pname

This decision was made in a902d99422 but
just for LLVM 12. Now we apply it consitently.

pkgs/development/compilers/llvm/6/default.nix

@@ -92,9 +92,9 @@ let
 
     libcxxStdenv = overrideCC stdenv buildLlvmTools.libcxxClang;
 
-    libcxx = callPackage ./libc++ {};
+    libcxx = callPackage ./libcxx {};
 
-    libcxxabi = callPackage ./libc++abi {};
+    libcxxabi = callPackage ./libcxxabi {};
 
     openmp = callPackage ./openmp.nix {};
   });