[ASan][libc++] Correct (explicit) annotation size (#79292)

A quick examination suggests that the current code in the codebase does
not lead to incorrect annotations. However, the intention is for the
object after the function to be annotated in a way that only its
contents are unpoisoned and the rest is poisoned. This commit makes it
explicit and avoids potential issues in future.

In addition, I have implemented a few tests for a function that helped
me identify the specific argument value.

Notice: there is no known scenario where old code results in incorrect
annotation.

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