gn build: Add a template for calling write_cmake_config.py

No behavior change.

Differential Revision: https://reviews.llvm.org/D56487


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@350905 91177308-0d34-0410-b5e6-96231b3b80d8
13 files changed