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/9/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;

pkgs/development/compilers/llvm/9/libcxx/default.nix

@@ -3,7 +3,7 @@
 }:
 
 stdenv.mkDerivation {
-  pname = "libc++";
+  pname = "libcxx";
   inherit version;
 
   src = fetch "libcxx" "0d2bj5i6mk4caq7skd5nsdmz8c2m5w5anximl5wz3x32p08zz089";

pkgs/development/compilers/llvm/9/libcxxabi/default.nix

@@ -3,7 +3,7 @@
 }:
 
 stdenv.mkDerivation {
-  pname = "libc++abi";
+  pname = "libcxxabi";
   inherit version;
 
   src = fetch "libcxxabi" "1b4aiaa8cirx52vk2p5kfk57qmbqf1ipb4nqnjhdgqps9jm7iyg8";