Develop policy for adopting LLVM minor version updates

It looks like we have an inconsistent approach to adopting LLVM/clang minor versions. We should likely revisit that.

Query: https://github.com/search?q=repo%3Adotnet%2Fdotnet-buildtools-prereqs-docker%20LLVM_VERSION&type=code

Ideally, we'd move forward with minor versions to ensure we pick up any security of performance updates. We also would need a way to validate those updates with runtime before publishing. We likely do that with some fancy (with some definition of that term) `manifest.json` edits to produce a new tag name for a temporary period.

@sbomer