[analyzer] Don't even try to convert floats to booleans for now.

We now have symbols with floating-point type to make sure that
(double)x == (double)x comes out true, but we still can't do much with
these. For now, don't even bother trying to create a floating-point zero
value; just give up on conversion to bool.

PR14634, C++ edition.

llvm-svn: 190953