gn build: s/root_out_dir/root_build_dir/g in llvm/utils/gn/build/write_cmake_config.gni.

This makes the generated files go to the right place when using a non-default toolchain.

Differential Revision: https://reviews.llvm.org/D56427

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@350963 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/utils/gn/build/write_cmake_config.gni b/utils/gn/build/write_cmake_config.gni
index 0c89166..6dca026 100644
--- a/utils/gn/build/write_cmake_config.gni
+++ b/utils/gn/build/write_cmake_config.gni
@@ -38,8 +38,8 @@
     ]
     args = [
              "-o",
-             rebase_path(outputs[0], root_out_dir),
-             rebase_path(sources[0], root_out_dir),
+             rebase_path(outputs[0], root_build_dir),
+             rebase_path(sources[0], root_build_dir),
            ] + invoker.values
 
     forward_variables_from(invoker,