[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.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@190953 91177308-0d34-0410-b5e6-96231b3b80d8