[clang][dataflow] Add `Environment::allows()`. (#70046)

This allows querying whether, given the flow condition, a certain
formula still
has a solution (though it is not necessarily implied by the flow
condition, as
`flowConditionImplies()` would check).

This can be checked today, but only with a double negation, i.e. to
check
whether, given the flow condition, a formula F has a solution, you can
check
`!Env.flowConditionImplies(Arena.makeNot(F))`. The double negation makes
this
hard to reason about, and it would be nicer to have a way of directly
checking
this.

For consistency, this patch also renames `flowConditionImplies()` to
`proves()`;
the old name is kept around for compatibility but deprecated.