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

[Vy Nguyen via llvm-commits]

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

Background: 

Telemetry code isn't always built (controlled by this LLVM_BUILD_TELEMETRY cmake flag)
This means users of the library may not have the library. So we're definding the `-DLLVM_BUILD_TELEMETRY` to be used in ifdef.

>From 008537373cd928fea9ac04b1aed39c2ecafec65a Mon Sep 17 00:00:00 2001
From: Vy Nguyen <vyng at google.com>
Date: Tue, 11 Feb 2025 10:36:06 -0500
Subject: [PATCH] Define -DLLVM_BUILD_TELEMETRY to be used in ifdef (by users
 of Telemetry)

---
 llvm/lib/CMakeLists.txt | 1 +
 1 file changed, 1 insertion(+)

diff --git a/llvm/lib/CMakeLists.txt b/llvm/lib/CMakeLists.txt
index d0a2bc9294381..08e3d7ddb8c12 100644
--- a/llvm/lib/CMakeLists.txt
+++ b/llvm/lib/CMakeLists.txt
@@ -43,6 +43,7 @@ add_subdirectory(TargetParser)
 add_subdirectory(TextAPI)
 if (LLVM_BUILD_TELEMETRY)
   add_subdirectory(Telemetry)
+  add_definitions(-DLLDB_BUILD_TELEMETRY)
 endif()
 add_subdirectory(ToolDrivers)
 add_subdirectory(XRay)