[llvm] Define -DLLVM_BUILD_TELEMETRY to be used in ifdef (PR #126746)

[Vy Nguyen via llvm-commits]

oontvoo wrote:

> How does this relate to #126715 (which defines the macro in lldb code)?
>
I thought it might be better to define the macro at llvm level (because there might be other users of llvm::Telemetry, in which case, we don't want to have them defining different macros to mean the same thing)



https://github.com/llvm/llvm-project/pull/126746