implicitboolconversion: warn about implicit conversion of call args to bool

...to be able to find problems like 6e0bdf04add338b7d5b29fc7b3fc9f08cfd5e96f
"sal_Bool arg of SetUseImagesInMenus was abused to squeeze '2' through it"
earlier when converting occurrences of sal_Bool to bool.

Restricting this check to function call arguments avoids too much noise while
hopefully still catching all the relevant problems.

(This check partially overlaps the pointertobool check, so implicit conversions
from pointers to bool call arguments will now generate two loplugin warnings,
but that's harmless.)

Change-Id: I0b03b1d1615aaf8bc18e7a84c56fff3ef9903508