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/5/default.nix

@@ -91,9 +91,9 @@ let
 
     libcxxStdenv = overrideCC stdenv buildLlvmTools.libcxxClang;
 
-    libcxx = callPackage ./libc++ {};
+    libcxx = callPackage ./libcxx {};
 
-    libcxxabi = callPackage ./libc++abi {};
+    libcxxabi = callPackage ./libcxxabi {};
 
     openmp = callPackage ./openmp.nix {};
   });

pkgs/development/compilers/llvm/5/libcxx/default.nix

@@ -1,7 +1,7 @@
 { lib, stdenv, fetch, cmake, python3, libcxxabi, fixDarwinDylibNames, version }:
 
 stdenv.mkDerivation {
-  pname = "libc++";
+  pname = "libcxx";
   inherit version;
 
   src = fetch "libcxx" "1672aaf95fgy4xsfra8pw24f6r93zwzpan1033hkcm8p2glqipvf";

pkgs/development/compilers/llvm/5/libcxxabi/default.nix

@@ -1,7 +1,7 @@
 { lib, stdenv, cmake, fetch, libcxx, libunwind, llvm, version }:
 
 stdenv.mkDerivation {
-  pname = "libc++abi";
+  pname = "libcxxabi";
   inherit version;
 
   src = fetch "libcxxabi" "12lp799rskr4fc2xr64qn4jfkjnfd8b1aymvsxyn4k9ar7r9pgqv";