QObject: use per-thread storage for qFlagLocation()

qFlagLocation() uses a global char* array to transport source location
information from the connect() side to the metaobject side. The size
of the array is 2 (two), which just about suffices for a single connect()
statement.

Obviously, if more than one thread makes a (_any_) connection at the same
time, the data is useless and, worse, there's a data race.

The non-reentrancy of qFlagLocations() cannot and need not be fixed,
but use a per-thread flagged_locations array in QThreadData so threads
don't disturb each other.

Task-number: QTBUG-3680

Change-Id: If1797c60751f551694def69afee6fbe295bbe2d2
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
(cherry picked from qtbase/c012ee2940bc087720b4aa0d257540921cf9a139)
Reviewed-by: Lars Knoll <lars.knoll@digia.com>