Check whether `ProgramPoint`s are useful without `LocationContext` / `StackFrame`

The type `ProgramPoint` is (AFAIK) only used by the clang static analyzer, where it corresponds to a concrete `LocationContext` [1], but there is a comment in `ProgramPoint.h` which claims that

```c++
// The StackFrame could be NULL to allow ProgramPoint to be used in
// context insensitive analysis.
```

@tigbr As we discussed in person and on [a review comment](https://github.com/llvm/llvm-project/pull/198211#discussion_r3305510757), please determine whether this comment is obsolete, and if it is obsolete, then remove/replace it.

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[1] The class `LocationContext` is being renamed to `StackFrame` right now.