This should be simpler to follow now.

Fix autodetection to return not only the newly detected toolchains,
but also those that are re-detected (taking their definition from
the alreadyKnown list passed to the autodetect method where possible).

This avoids running lots of toolchains during start-up, but still
enables us to fix QTCREATORBUG-12751

Task-number: QTCREATORBUG-12751
Change-Id: Ie74e7cffb2b014a6132cc8559db232397344f2f1
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>