Split up function qMakeFieldInfo() in ODBC plugin.

Refactor code to extract an overload of qMakeFieldInfo()
that only takes an SQLHANDLE hStmt parameter. This allows
for easy debugging and inspecting driver-dependent additional
columns in QODBCDriver::record().

Task-number: QTBUG-39388
Change-Id: Id338838f837746beff1b2711bf2cd66ca8c7afe6
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>
Retrieve auto-value for ODBC fields.

Introduce a static function returning the value called from
qMakeFieldInfo(SQLHANDLE). The field is currently only populated
when executing a query. Populating it from QODBCDriver::record()
would require executing a dummy query, which is problematic since
QSqlField does not have any provisions for delayed evaluation.
Document the limitation.

Task-number: QTBUG-39388
Change-Id: Ib2d2f2653b8b757389f627142c61c13a117fef72
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>