Move DEBUG_TYPE below LLVM headers (#44997)
Some LLVM headers set DEBUG_TYPE internally (and undef it later),
causing build failures with the newly added STATISTICs. Fix that
by moving DEBUG_TYPE below the headers.
[1] https://github.com/llvm/llvm-project/blob/09c2b7c35af8c4bad39f03e9f60df8bd07323028/llvm/include/llvm/Support/GenericDomTreeConstruction.h#L49
