[llvm] [cmake] Expose LLVM_BUILD_TELEMETRY in LLVMConfig.cmake (#126710)

Add `LLVM_BUILD_TELEMETRY` to the list of flags exposed in
`LLVMConfig.cmake`. This fixes telemetry library being misdetected as
`OFF` when building LLDB standalone.

Fixes bac62ee.

------

I guess this also needs a backport to 20.x.

Author: mgorny

llvm/cmake/modules/LLVMConfig.cmake.in

@@ -100,6 +100,8 @@ set(LLVM_ENABLE_PIC @LLVM_ENABLE_PIC@)
 
 set(LLVM_BUILD_32_BITS @LLVM_BUILD_32_BITS@)
 
+set(LLVM_BUILD_TELEMETRY @LLVM_BUILD_TELEMETRY@)
+
 if (NOT "@LLVM_PTHREAD_LIB@" STREQUAL "")
   set(LLVM_PTHREAD_LIB "@LLVM_PTHREAD_LIB@")
 endif()