[LifetimeSafety] Implement dataflow analysis for loan propagation (#148065) This patch introduces the core dataflow analysis infrastructure for the C++ Lifetime Safety checker. This change implements the logic to propagate "loan" information across the control-flow graph. The primary goal is to compute a fixed-point state that accurately models which pointer (Origin) can hold which borrow (Loan) at any given program point. Key components * `LifetimeLattice`: Defines the dataflow state, mapping an `OriginID` to a `LoanSet` using `llvm::ImmutableMap`. * `Transferer`: Implements the transfer function, which updates the `LifetimeLattice` by applying the lifetime facts (Issue, AssignOrigin, etc.) generated for each basic block. * `LifetimeDataflow`: A forward dataflow analysis driver that uses a worklist algorithm to iterate over the CFG until the lattice state converges. The existing test suite has been extended to check the final dataflow results. This work is a prerequisite for the final step of the analysis: consuming these results to identify and report lifetime violations.
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