[clang][dataflow] Add function to `WatchedLiteralsSolver` that reports whether...

[clang][dataflow] Add function to `WatchedLiteralsSolver` that reports whether the iteration limit has been reached.

This change provides a centralized record of whether the solver is
"exhausted". In general, once the solver is exhausted, further uses do not
produce meaningful conclusions. Clients can use this information, for example,
to choose to discard all results based on the solver, instead of trying to
distinguish those reached before the limit was reached. Providing it at in the
solver avoids the need to propagate it from each callsite to the client of the
solver.

Differential Revision: https://reviews.llvm.org/D155636