[clang][dataflow] Optimize flow condition representation

Enable efficient implementation of context-aware joining of distinct
boolean values. It can be used to join distinct boolean values while
preserving flow condition information.

Flow conditions are represented as Token <=> Clause iff formulas. To
perform context-aware joining, one can simply add the tokens of flow
conditions to the formula when joining distinct boolean values, e.g:
`makeOr(makeAnd(FC1, Val1), makeAnd(FC2, Val2))`. This significantly
simplifies the implementation of `Environment::join`.

This patch removes the `DataflowAnalysisContext::getSolver` method.
The `DataflowAnalysisContext::flowConditionImplies` method should be
used instead.

Reviewed-by: ymandel, xazax.hun

Differential Revision: https://reviews.llvm.org/D124395