Use configure depends to trigger reconfiguration when LLVMBuild files change

Summary:
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).

Reviewers: theraven

Reviewed By: theraven

Subscribers: theraven, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D80218