[clang][dataflow] Use smart pointer caching in unchecked optional accessor (#120249)

Part 2 (and final part) following
https://github.com/llvm/llvm-project/pull/120102
Allows users to do things li

[clang][dataflow] Use smart pointer caching in unchecked optional accessor (#120249)

Part 2 (and final part) following
https://github.com/llvm/llvm-project/pull/120102
Allows users to do things like:

```
if (o->x.has_value()) {
((*o).x).value();
}
```
where the `->` and `*` are operator overload calls.

A user could instead extract the nested optional into a local variable
once instead of doing two accessor calls back to back, but currently
they are unsure why the code is flagged.

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[clang][dataflow] Add a lattice to help cache const accessor methods (#111006)

By caching const accessor methods we can sometimes treat method call
results as stable (e.g., for issue
https://githu

[clang][dataflow] Add a lattice to help cache const accessor methods (#111006)

By caching const accessor methods we can sometimes treat method call
results as stable (e.g., for issue
https://github.com/llvm/llvm-project/issues/58510). Users can clear the
cache when a non-const method is called that may modify the state of an
object.
This is represented as mixin.

It will be used in a follow on patch to change
bugprone-unchecked-optional-access's lattice from NoopLattice to
CachedConstAccessorsLattice<NoopLattice>, along with some additional
transfer functions.

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