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This patch follows the reversion of #107279, which caused errors by including `llvm/Config/config.h` in a public header. In order to reapply that patch, this PR moves the definition of the config option to the `llvm-config.h` header instead, so that it can be used publicly. This patch also adds `LLVM_` as a prefix to the define, since it is now in a public header.
See docs/CMake.html for instructions on how to build LLVM with CMake.