C++: Add Taint through int -> bool casts

[MathiasVP]

The final step in undoing the workaround in https://github.com/github/codeql/pull/14886/files#diff-40e2016a464c1cf96ccb2a7469f1d6d2da1768ab2f355c8beb3bd0050ecd6a18R31 we add taint-flow through the implicit integer to boolean cast generated in a snippet such as:

int x = some_int();
if(!x) { ... }

This is necessary because we generate IR that's equivalent to:

int x = some_int();
if(!(x != 0)) { ... }

and since we already have taint-flow from x to !x, the additional taint from x to x != 0 ensures that we get flow to the result of the condition.

[MathiasVP]

DCA looks uneventful 🎉