[clang] [clang][dataflow] Add an early-out to `flowConditionImplies()` / `flowConditionAllows()`. (PR #77453)

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martinboehme wrote:

Closing the loop: My logic for `flowConditionAllows()` was wrong. If the flow condition is contradictory, `flowConditionAllows(true) == false`, so the early-out was wrong.

https://github.com/llvm/llvm-project/pull/77453