[TableGen] CodeEmitterGen - emit report_fatal_error(const char*) instead of...

[TableGen] CodeEmitterGen - emit report_fatal_error(const char*) instead of report_fatal_error(std::string&)

As described on D111049, we're trying to remove the <string> dependency from error handling. In most cases the plan is to use the Twine() variant directly but to reduce introducing additional headers for the generated files, I'm using the const char* variant here instead.