[llvm] [llvm] [cmake] Expose `LLVM_BUILD_TELEMETRY` in `LLVMConfig.cmake` (PR #126710)

[Nikita Popov via llvm-commits]

https://github.com/nikic approved this pull request.

LGTM

> I guess this also needs a backport to 20.x.

I don't think lldb telemetry is part of 20.x.

https://github.com/llvm/llvm-project/pull/126710