[clang][dataflow] Add an early-out to `flowConditionImplies()` / `flowConditionAllows()`. (#78172)

This saves having to assemble the set of constraints and run the SAT
solver in
the trivial case of `flowConditionImplies(true)` or
`flowConditionAllows(false)`.

This is an update / reland of my previous reverted
[#77453](https://github.com/llvm/llvm-project/pull/77453). That PR
contained a
logic bug -- the early-out for `flowConditionAllows()` was wrong because
my
intuition about the logic was wrong. (In particular, note that
`flowConditionImplies(F)` does not imply `flowConditionAllows(F)`, even
though
this may run counter to intuition.)

I've now done what I should have done on the first iteration and added
more
tests. These pass both with and without my early-outs.

This patch is a performance win on the benchmarks for the Crubit
nullability
checker, except for one slight regression on a relatively short
benchmark:

```
name                              old cpu/op   new cpu/op   delta
BM_PointerAnalysisCopyPointer     68.5µs ± 7%  67.6µs ± 4%    ~     (p=0.159 n=18+19)
BM_PointerAnalysisIntLoop          173µs ± 3%   162µs ± 4%  -6.40%  (p=0.000 n=19+20)
BM_PointerAnalysisPointerLoop      307µs ± 2%   312µs ± 4%  +1.56%  (p=0.013 n=18+20)
BM_PointerAnalysisBranch           199µs ± 4%   181µs ± 4%  -8.81%  (p=0.000 n=20+20)
BM_PointerAnalysisLoopAndBranch    503µs ± 3%   508µs ± 2%    ~     (p=0.081 n=18+19)
BM_PointerAnalysisTwoLoops         304µs ± 4%   286µs ± 2%  -6.04%  (p=0.000 n=19+20)
BM_PointerAnalysisJoinFilePath    4.78ms ± 3%  4.54ms ± 4%  -4.97%  (p=0.000 n=20+20)
BM_PointerAnalysisCallInLoop      3.05ms ± 3%  2.90ms ± 4%  -5.05%  (p=0.000 n=19+20)
```

When running clang-tidy on real-world code, the results are less clear.
In
three runs, averaged, on an arbitrarily chosen input file, I get 11.60 s
of user
time without this patch and 11.40 s with it, though with considerable
measurement noise (I'm seeing up to 0.2 s of variation between runs).

Still, this is a very simple change, and it is a clear win in
benchmarks, so I
think it is worth making.