[clang][dataflow] Add widening API and implement it for built-in boolean model.

* Adds API support for widening of lattice elements and environments,
* Updates the algorithm to apply widening where appropriate,
* Implements widening for boolean values. In the process, moves the unsoundness
  of comparison from the default implementation of
  `Environment::ValueModel::compare` to model-specific handling inside
  `DataflowEnvironment::equivalentTo`. This change is intended to clarify
  the source and location of unsoundess.

This patch is a replacement for, and was based substantially on, https://reviews.llvm.org/D131645.

Differential Revision: https://reviews.llvm.org/D137948