WaE: VarDecl, use "bool" instead of "sal_Bool" [loplugin:salbool]

This likely is a false warning or over-enthusiasm in the salbool plug-in, but
on the other hand, I don't understand why the recently added case needs to use
different coding idioms from the preceding ones in the function? So make the
code more uniform.

Change-Id: I518022c59f6bd54740676cba95670e78b9a40d19