C++: Add MaD models for SysAllocString and friends

[MathiasVP]

The lack of these models were causing some false negatives at Microsoft.

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[redsun82]

LGTM!

[jketema]

These two presumably can cut the tail of the string passed in as an argument (when the passed length is too short). I assume we still consider that value flow (although part of the data has gone missing)?

[MathiasVP]

That's a good point. We model that somewhat inconsistency across different models. For example, strncpy is currently modeled as taint because the length argument could mean that we only did a partial copy. In practice, I have a feeling that most of these length-bounded functions are used simply as defensive programming and do provide full copies in most cases. So nowadays I tend to lean towards modeling them as value-preserving.