Use configure depends to trigger reconfiguration when LLVMBuild files change
The existing logic has a workaround where configure_file is used to write a single dummy file output many times.
CMake has a feature to more directly add the dependency and avoid the dummy file (it is available in the minimum version specified).
Reviewed By: theraven
Subscribers: theraven, llvm-commits
Differential Revision: https://reviews.llvm.org/D80218