[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 think that defining some sort of a macro in llvm code makes sense, but this probably not the way to do that. Docs for `add_definitions` say (emphasis mine):
> Adds definitions to the compiler command line for targets in the current directory, whether added before or after this command is invoked, and for the ones in *sub-directories added after*.

I.e.,, this creates a very inconsistent state where e.g. `XRay` has the macro defined, but `TextAPI` does not. This should probably go to [llvm-config.h](https://github.com/llvm/llvm-project/blob/main/llvm/include/llvm/Config/llvm-config.h.cmake) instead.

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