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

[Pavel Labath via llvm-commits]

labath 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)

Makes sense.

> Done - added the def to config.h.

In that case, could you also delete the definition in llvm/lib/CMakeLists.txt and the one introduced in #126715 ?

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