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/8/default.nix

@@ -180,12 +180,12 @@ let
 
     libcxxStdenv = overrideCC stdenv buildLlvmTools.libcxxClang;
 
-    libcxx = callPackage ./libc++ ({} //
+    libcxx = callPackage ./libcxx ({} //
       (lib.optionalAttrs (stdenv.hostPlatform.useLLVM or false) {
         stdenv = overrideCC stdenv buildLlvmTools.lldClangNoLibcxx;
       }));
 
-    libcxxabi = callPackage ./libc++abi ({} //
+    libcxxabi = callPackage ./libcxxabi ({} //
       (lib.optionalAttrs (stdenv.hostPlatform.useLLVM or false) {
         stdenv = overrideCC stdenv buildLlvmTools.lldClangNoLibcxx;
         libunwind = libraries.libunwind;