[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 bac62ee5b473e70981a6bd9759ec316315fca07d.
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I guess this also needs a backport to 20.x.