[clang][dataflow] Enable comparison of distinct values in Environment

Make specializations of `DataflowAnalysis` extendable with domain-specific
logic for comparing distinct values when comparing environments.

This includes a breaking change to the `runDataflowAnalysis` interface
as the return type is now `llvm::Expected<...>`.

This is part of the implementation of the dataflow analysis framework.
See "[RFC] A dataflow analysis framework for Clang AST" on cfe-dev.

Reviewed-by: ymandel, xazax.hun

Differential Revision: https://reviews.llvm.org/D118596