Quick fix to unbreak tblgen past 8ae18303

llvm/tools/llvm-objdump/llvm-objdump.cpp:128:38: error: constexpr variable 'ObjdumpInfoTable' must be initialized by a constant expression
static constexpr opt::OptTable::Info ObjdumpInfoTable[] = {
                                     ^                    ~
ObjdumpOpts.inc:30:45: note: non-constexpr function 'substr' cannot be used in a constant expression
OPTION(prefix_0, llvm::StringRef("<input>").substr(0), INPUT, Input, INVALID, INVALID, nullptr, 0, 0, nullptr, nullptr, nullptr)
                                            ^