clang: only link if LLVMgold exists

buildEnv now errors with broken symlinks. Lots of things like this
still exist.

Discussion in #39038.

(cherry picked from commit 7b73c7f49a)

pkgs/development/compilers/llvm/3.4/clang.nix

@@ -29,7 +29,10 @@ stdenv.mkDerivation {
   # Clang expects to find LLVMgold in its own prefix
   # Clang expects to find sanitizer libraries in its own prefix
   postInstall = ''
-    ln -sv ${llvm}/lib/LLVMgold.so $out/lib
+    if [ -e ${llvm}/lib/LLVMgold.so ]; then
+      ln -sv ${llvm}/lib/LLVMgold.so $out/lib
+    fi
+
     ln -sv ${llvm}/lib/clang/${version}/lib $out/lib/clang/${version}/
   '';
 

pkgs/development/compilers/llvm/3.5/clang.nix

@@ -31,7 +31,10 @@ in stdenv.mkDerivation {
   # Clang expects to find LLVMgold in its own prefix
   # Clang expects to find sanitizer libraries in its own prefix
   postInstall = ''
-    ln -sv ${llvm}/lib/LLVMgold.so $out/lib
+    if [ -e ${llvm}/lib/LLVMgold.so ]; then
+      ln -sv ${llvm}/lib/LLVMgold.so $out/lib
+    fi
+
     ln -sv ${llvm}/lib/clang/${version}/lib $out/lib/clang/${version}/
     ln -sv $out/bin/clang $out/bin/cpp
   '';

pkgs/development/compilers/llvm/3.7/clang/default.nix

@@ -32,7 +32,10 @@ let
     # Clang expects to find LLVMgold in its own prefix
     # Clang expects to find sanitizer libraries in its own prefix
     postInstall = ''
-      ln -sv ${llvm}/lib/LLVMgold.so $out/lib
+      if [ -e ${llvm}/lib/LLVMgold.so ]; then
+        ln -sv ${llvm}/lib/LLVMgold.so $out/lib
+      fi
+
       ln -sv ${llvm}/lib/clang/${version}/lib $out/lib/clang/${version}/
       ln -sv $out/bin/clang $out/bin/cpp
     '';

pkgs/development/compilers/llvm/3.8/clang/default.nix

@@ -34,7 +34,10 @@ let
     # Clang expects to find LLVMgold in its own prefix
     # Clang expects to find sanitizer libraries in its own prefix
     postInstall = ''
-      ln -sv ${llvm}/lib/LLVMgold.so $out/lib
+      if [ -e ${llvm}/lib/LLVMgold.so ]; then
+        ln -sv ${llvm}/lib/LLVMgold.so $out/lib
+      fi
+
       ln -sv ${llvm}/lib/clang/${version}/lib $out/lib/clang/${version}/
       ln -sv $out/bin/clang $out/bin/cpp
 

pkgs/development/compilers/llvm/3.9/clang/default.nix

@@ -36,7 +36,10 @@ let
     # Clang expects to find LLVMgold in its own prefix
     # Clang expects to find sanitizer libraries in its own prefix
     postInstall = ''
-      ln -sv ${llvm}/lib/LLVMgold.so $out/lib
+      if [ -e ${llvm}/lib/LLVMgold.so ]; then
+        ln -sv ${llvm}/lib/LLVMgold.so $out/lib
+      fi
+
       ln -sv ${llvm}/lib/clang/${version}/lib $out/lib/clang/${version}/
       ln -sv $out/bin/clang $out/bin/cpp
 

pkgs/development/compilers/llvm/4/clang/default.nix

@@ -52,7 +52,10 @@ let
     # Clang expects to find LLVMgold in its own prefix
     # Clang expects to find sanitizer libraries in its own prefix
     postInstall = ''
-      ln -sv ${llvm}/lib/LLVMgold.so $out/lib
+      if [ -e ${llvm}/lib/LLVMgold.so ]; then
+        ln -sv ${llvm}/lib/LLVMgold.so $out/lib
+      fi
+
       ln -sv ${llvm}/lib/clang/${release_version}/lib $out/lib/clang/${release_version}/
       ln -sv $out/bin/clang $out/bin/cpp
 

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

@@ -53,7 +53,10 @@ let
     # Clang expects to find LLVMgold in its own prefix
     # Clang expects to find sanitizer libraries in its own prefix
     postInstall = ''
-      ln -sv ${llvm}/lib/LLVMgold.so $out/lib
+      if [ -e ${llvm}/lib/LLVMgold.so ]; then
+        ln -sv ${llvm}/lib/LLVMgold.so $out/lib
+      fi
+
       ln -sv ${llvm}/lib/clang/${release_version}/lib $out/lib/clang/${release_version}/
       ln -sv $out/bin/clang $out/bin/cpp