[clang][dataflow] Add support for a Top value in boolean formulas.

Currently, our boolean formulas (`BoolValue`) don't form a lattice, since they
have no Top element. This patch adds such an element, thereby "completing" the
built-in model of bools to be a proper semi-lattice. It still has infinite
height, which is its own problem, but that can be solved separately, through
widening and the like.

Patch 1 for Issue #56931.

Differential Revision: https://reviews.llvm.org/D135397