avoid deadlock in cache() during spec/cache loading

avoid deadlock in cache() during spec/cache loading

sync up; doesn't do anything in lupdate.

if we end up calling cache() from within the initialization of the base
context, we cannot wait for for the completion of that initialization
before we proceed, obviously.

Change-Id: If30c6f3665fe423e767373a8821c406b2f5e0eca
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
Reviewed-by: Daniel Teske <daniel.teske@digia.com>
(cherry picked from qtcreator/b64b4431c20afd9e39c1463e736f998ef450688f)