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

[Michał Górny via llvm-commits]

mgorny wrote:

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

Well, I was referring to the fact that https://github.com/llvm/llvm-project/commit/bac62ee5b473e70981a6bd9759ec316315fca07d mentioned backporting the flag, and I believe this to be reasonable part of the flag, even if it weren't really used inside LLVM project at the time.

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