Similar to #91329, `mlir-pdll` is a tool used in tablegen macros that unregisters from common flags, including `-D` macros. Because a macro may be used globally, e.g. configured via `LLVM_TABLEGEN_FLAGS`, we want this tool to just ignore the macro instead of a fatal failure due to the unrecognized flag.
Multi-Level Intermediate Representation
See https://mlir.llvm.org/ for more information.