loplugin:implicitboolconversion: warn about conversions to unsigned char

...while avoiding warnings about conversions to bool-like typedefs (sal_Bool
etc.), also in cases where those typedefs are used as type arguments of
template specializations (which is no little feat, and the current code is only
an approximation of it, one that appears to cover the relevant cases in our code
base though).

Change-Id: I0ed3801aec7787bf38b429b66e25244ec00cac9b