llvmPackages_*.clang: pick clangUseLLVM if targetPlatform.useLLVM

libcxxClang still depends on cc wrapper's gccForLibs for libgcc which is
not available when useLLVM is set. In such cases we need to switch to
clangUseLLVM and (try) to use compiler-rt instead.

Resolves #153759: pkgsLLVM.llvmPackages.stdenv now correctly
clangUseLLVM as cc, allowing compilation to work as expected.

pkgs/development/compilers/llvm/10/default.nix

@@ -91,8 +91,8 @@ let
 
     # pick clang appropriate for package set we are targeting
     clang =
-      if (pkgs.targetPackages.stdenv or stdenv).cc.isGNU
-      then tools.libstdcxxClang
+      /**/ if stdenv.targetPlatform.useLLVM or false then tools.clangUseLLVM
+      else if (pkgs.targetPackages.stdenv or stdenv).cc.isGNU then tools.libstdcxxClang
       else tools.libcxxClang;
 
     libstdcxxClang = wrapCCWith rec {

pkgs/development/compilers/llvm/11/default.nix

@@ -106,8 +106,8 @@ let
 
     # pick clang appropriate for package set we are targeting
     clang =
-      if (pkgs.targetPackages.stdenv or stdenv).cc.isGNU
-      then tools.libstdcxxClang
+      /**/ if stdenv.targetPlatform.useLLVM or false then tools.clangUseLLVM
+      else if (pkgs.targetPackages.stdenv or stdenv).cc.isGNU then tools.libstdcxxClang
       else tools.libcxxClang;
 
     libstdcxxClang = wrapCCWith rec {

pkgs/development/compilers/llvm/12/default.nix

@@ -94,8 +94,8 @@ let
 
     # pick clang appropriate for package set we are targeting
     clang =
-      if (pkgs.targetPackages.stdenv or stdenv).cc.isGNU
-      then tools.libstdcxxClang
+      /**/ if stdenv.targetPlatform.useLLVM or false then tools.clangUseLLVM
+      else if (pkgs.targetPackages.stdenv or stdenv).cc.isGNU then tools.libstdcxxClang
       else tools.libcxxClang;
 
     libstdcxxClang = wrapCCWith rec {

pkgs/development/compilers/llvm/13/default.nix

@@ -95,8 +95,8 @@ let
 
     # pick clang appropriate for package set we are targeting
     clang =
-      if (pkgs.targetPackages.stdenv or stdenv).cc.isGNU
-      then tools.libstdcxxClang
+      /**/ if stdenv.targetPlatform.useLLVM or false then tools.clangUseLLVM
+      else if (pkgs.targetPackages.stdenv or stdenv).cc.isGNU then tools.libstdcxxClang
       else tools.libcxxClang;
 
     libstdcxxClang = wrapCCWith rec {

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

@@ -67,8 +67,8 @@ let
 
     # pick clang appropriate for package set we are targeting
     clang =
-      if (pkgs.targetPackages.stdenv or stdenv).cc.isGNU
-      then tools.libstdcxxClang
+      /**/ if stdenv.targetPlatform.useLLVM or false then tools.clangUseLLVM
+      else if (pkgs.targetPackages.stdenv or stdenv).cc.isGNU then tools.libstdcxxClang
       else tools.libcxxClang;
 
     libstdcxxClang = wrapCCWith rec {

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

@@ -68,8 +68,8 @@ let
 
     # pick clang appropriate for package set we are targeting
     clang =
-      if (pkgs.targetPackages.stdenv or stdenv).cc.isGNU
-      then tools.libstdcxxClang
+      /**/ if stdenv.targetPlatform.useLLVM or false then tools.clangUseLLVM
+      else if (pkgs.targetPackages.stdenv or stdenv).cc.isGNU then tools.libstdcxxClang
       else tools.libcxxClang;
 
     libstdcxxClang = wrapCCWith rec {

pkgs/development/compilers/llvm/7/default.nix

@@ -98,8 +98,8 @@ let
 
     # pick clang appropriate for package set we are targeting
     clang =
-      if (pkgs.targetPackages.stdenv or stdenv).cc.isGNU
-      then tools.libstdcxxClang
+      /**/ if stdenv.targetPlatform.useLLVM or false then tools.clangUseLLVM
+      else if (pkgs.targetPackages.stdenv or stdenv).cc.isGNU then tools.libstdcxxClang
       else tools.libcxxClang;
 
     libstdcxxClang = wrapCCWith rec {

pkgs/development/compilers/llvm/8/default.nix

@@ -99,8 +99,8 @@ let
 
     # pick clang appropriate for package set we are targeting
     clang =
-      if (pkgs.targetPackages.stdenv or stdenv).cc.isGNU
-      then tools.libstdcxxClang
+      /**/ if stdenv.targetPlatform.useLLVM or false then tools.clangUseLLVM
+      else if (pkgs.targetPackages.stdenv or stdenv).cc.isGNU then tools.libstdcxxClang
       else tools.libcxxClang;
 
     libstdcxxClang = wrapCCWith rec {

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

@@ -99,8 +99,8 @@ let
 
     # pick clang appropriate for package set we are targeting
     clang =
-      if (pkgs.targetPackages.stdenv or stdenv).cc.isGNU
-      then tools.libstdcxxClang
+      /**/ if stdenv.targetPlatform.useLLVM or false then tools.clangUseLLVM
+      else if (pkgs.targetPackages.stdenv or stdenv).cc.isGNU then tools.libstdcxxClang
       else tools.libcxxClang;
 
     libstdcxxClang = wrapCCWith rec {

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

@@ -95,8 +95,8 @@ let
 
     # pick clang appropriate for package set we are targeting
     clang =
-      if (pkgs.targetPackages.stdenv or stdenv).cc.isGNU
-      then tools.libstdcxxClang
+      /**/ if stdenv.targetPlatform.useLLVM or false then tools.clangUseLLVM
+      else if (pkgs.targetPackages.stdenv or stdenv).cc.isGNU then tools.libstdcxxClang
       else tools.libcxxClang;
 
     libstdcxxClang = wrapCCWith rec {