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
1 file changed