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

[Donát Nagy via cfe-commits]

NagyDonat wrote:

> It is full of sphagetti code and details like e.g. that `SwitchNodeBuilder` is not part of the class hierarchy derived from the base class `NodeBuilder` 🫣 .

Apparently I will need to change this – `CheckerContext::addTransition` and similar methods expect that the `ExprEngine` can provide an "active" `NodeBuilder` instance, so I need to ensure that `SwitchNodeBuilder` provides a `NodeBuilder`. 

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