Disable unsigned integer sanitizer for basic_string::replace()

[tomcherry]

Summary

basic_string::replace() has the below line

sz += n2 - __n1;

which fails overflow checks if n1 > n2, as the negative result
from the subtraction then overflows the original __sz when added to
it.

This behavior is valid as unsigned integer overflow is defined to wrap
around the maximum value and that produces the correct final value for
__sz. Therefore, we disable this check on this function.

[srhines]

You probably want to remove the Change-Id section above in your description (or at least drop that when you finally submit this to libc++).

[EricWF]

include/string

2559

Please put this comment inside one of the function bodies, right above the offending line.

I think putting it here will result in it maybe becoming unmaintained.

[EricWF]

Side note: There are plenty of tests in the test-suite that trigger this overflow, so no new tests are needed.

When I have time I'm going to enable -fsanitize=unsigned-integer-overflow once I have time to clean up any existing failures.

[tomcherry]

Moved comments to an appropriate line

[EricWF]

@Tom Do you have commit acces or would you like me to commit this for you?

[tomcherry]

I don't have commit access, so please commit for me.

Thank you

[EricWF]

Committed in r297355.