transpile: support __attribute__((cleanup(func)))

[thedataking]

Translate __attribute__((cleanup(func))) using a CleanupGuard<T> whose Drop calls the user's cleanup function on scope exit.

Closes #1814.

[fw-immunant]

Looks good overall, except for concerns around constructing reference to obtain raw pointer. I would have liked more context on several changes (e.g. the first which appears to be a longstanding bug for which a reproducer would be good to see) but they do check out.

[thedataking]

I would have liked more context on [...] the first which appears to be a longstanding bug

Ack on your overall point. For the first commit, it turns out that def can never be null (it is initialized to VD, which is never null in this function), so I changed it to simply remove the dead code. The null guard was most likely carried over from the VisitFunctionDecl handler, where def comes from getDefinition() and the null case is real.