[clang] [llvm] [analyzer] New checker: optin.core.UnconditionalVAArg (PR #175602)

[Donát Nagy via cfe-commits]

NagyDonat wrote:

I verified that the `BranchCondition` callback does not react to `switch` statements because although they perform branching, they don't have a branch _condition_ like the other statements.

Perhaps I could switch to using a `BlockEntrance` callback because I'm pretty confident that it does react to blocks entered via a `switch` statement. However there are also several other differences between the two callbacks:
1. `BlockEntrance` is activated by the transitions from the imaginary "entry" blocks of inlined functions to the first "real" block;
2. `BlockEntrance` is activated by the transitions from the final block of a function to the imaginary "exit" block;
3. `BlockEntrance` is activated at points where two or more paths (e.g. the `if` and the `else` branch) are merged;
4. I strongly suspect that `BlockEntrance` is activated when execution enters the body of a `do { ... } while (...)` loop;
5. `BlockEntrance` may be executed when the execution passes over a goto label (which is e.g. referenced by a `goto` later in the body of the code).
6. `BlockEntrance` may be executed when the execution encounters C++ exception handling logic.

For this checker difference No.3 is irrelevant (because it ignores everything after encountering a path split, and path merging cannot happen without a path split) and No.6 is either helpful or irrelevant (exceptions are real branching points, but they rarely appear in C-style variadic functions), but the No.1-2 (the imaginary entry/exit) must be handled somehow and No.4-5. (do-while, goto) are also drawbacks of the `BlockEntrance` callback for this usecase (although they are rare, so it is forgivable if the checker produces false negatives on them).

As another approach, I also thought about modifying the engine to let it trigger the `BranchCondition` callbacks upon entering `switch` statements. It would be feasible to implement this because there are very few checkers that use `BranchCondition` – but I feel that it would be against the "real meaning" of that callback.

@steakhal (or anybody else familiar with `BlockEntrance`)
- What do you think about this situation?
- Should I use the `BlockEntrance` callback or do you see another approach?
- If I use `BlockEntrance` do you know a straightforward way to filter 

https://github.com/llvm/llvm-project/pull/175602