[clang][dataflow] Add explicit "AST" nodes for implications and iff

Previously we used to desugar implications and biconditionals into
equivalent CNF/DNF as soon as possible. However, this desugaring makes
debug output (Environment::dump()) less readable than it could be.
Therefore, it makes sense to keep the sugared representation of a
boolean formula, and desugar it in the solver.

Reviewed By: sgatev, xazax.hun, wyt

Differential Revision: https://reviews.llvm.org/D130519