Relax comparison of qc_sqlite and qc_mysqlembedded results.

qc_get_field_info reports not just the fields but in which context they are used. Currently compare, the program using which the results of qc_sqlite and qc_mysqlembedded are compared treats it as an error if the usage reports differ.

That information is not used anywhere and is quite tricky to get right in the context of subselects and CTEs, so for now discrepancies should be reported as a warning.