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

[Vy Nguyen via llvm-commits]

oontvoo wrote:

> 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.

Done - added the def to config.h.

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