[llvm] Add CMake flag to compile out the telemetry framework (llvm#124850)

Add a CMake flag (LLVM_BUILD_TELEMETRY) to disable building the
telemetry framework. The flag being enabled does *not* mean that
telemetry is being collected, it merely means we're building the generic
telemetry framework. Hence the flag is enabled by default.

Motivated by this Discourse thread:
https://discourse.llvm.org/t/how-to-disable-building-llvm-clang-telemetry/84305

Author: JDevlieghere

llvm/CMakeLists.txt

@@ -829,6 +829,7 @@ option (LLVM_ENABLE_DOXYGEN "Use doxygen to generate llvm API documentation." OF
 option (LLVM_ENABLE_SPHINX "Use Sphinx to generate llvm documentation." OFF)
 option (LLVM_ENABLE_OCAMLDOC "Build OCaml bindings documentation." ON)
 option (LLVM_ENABLE_BINDINGS "Build bindings." ON)
+option (LLVM_BUILD_TELEMETRY "Build the telemtry library. This does not enable telemetry." ON)
 
 set(LLVM_INSTALL_DOXYGEN_HTML_DIR "${CMAKE_INSTALL_DOCDIR}/llvm/doxygen-html"
     CACHE STRING "Doxygen-generated HTML documentation install directory")

llvm/lib/CMakeLists.txt

@@ -41,7 +41,9 @@ add_subdirectory(ProfileData)
 add_subdirectory(Passes)
 add_subdirectory(TargetParser)
 add_subdirectory(TextAPI)
-add_subdirectory(Telemetry)
+if (LLVM_BUILD_TELEMETRY)
+  add_subdirectory(Telemetry)
+endif()
 add_subdirectory(ToolDrivers)
 add_subdirectory(XRay)
 if (LLVM_INCLUDE_TESTS)

llvm/unittests/CMakeLists.txt

@@ -63,7 +63,9 @@ add_subdirectory(Support)
 add_subdirectory(TableGen)
 add_subdirectory(Target)
 add_subdirectory(TargetParser)
-add_subdirectory(Telemetry)
+if (LLVM_BUILD_TELEMETRY)
+  add_subdirectory(Telemetry)
+endif()
 add_subdirectory(Testing)
 add_subdirectory(TextAPI)
 add_subdirectory(Transforms)