Fixed error message printing in write_cmake_config.py

Summary:
Previously, write_cmake_config.py would raise an error while printing
the error, because `leftovers` in "'\n'.join(leftovers)" is a tuple.

Subscribers: mgorny, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D60766

llvm-svn: 358557