Add a method for conditional deferral and use it in the new analyzer

[ilevkivskyi]

This idea appeared in #6724. It will be useful for plugins and in general to have something like this in SemanticAnalyzer:

def defer_if_possible(self) -> bool:
    if not self.final_iteration:
        self.defer()
        return True
    return False

Then it can be used like this:

if ctx.api.defer_if_possible():
    ...
else:
    ... # probably give some error

Maybe we can also add it to the old analyzer as always returning False, then we can completely hide the old vs new analyzer details in plugin API.

[JukkaL]

process_placeholder is enough for now, as discussed in #7159.