The current assertion failure messages produced by Hardening are not
very grep-friendly (the common part is rarther generic and requires
wildcards to match). While it's possible to use `__FILE__` for grepping,
it's easier and more straighforward to simply add a libc++-specific
prefix; this is especially important for the planned `observe` mode that
might produce many assertion failure messages over the course of the
program's execution that later need to be filtered and examined.
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