C++: Create a `ZeroBound` per function

[MathiasVP]

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When we call ZeroBound::getExpr to get an expression that matches a given bound we eventually reach https://github.com/github/codeql/blob/8b8e74cc9a11f3cb5bedd0aed42ec26de179dc61/cpp/ql/lib/experimental/semmle/code/cpp/rangeanalysis/Bound.qll#L57 which will match any instruction with a constant value of 0. As @rdmarsh2 noted, this gives a very large tuple explosion.

This PR creates a per-function ZeroBound and restricts the expressions returned by ZeroBound::getExpr to be only instructions in the same callable as the ZeroBound.

[aschackmull]

Why is this relevant? Doesn't this just indicate that you have a poor join-order somewhere else?

[MathiasVP]

Why is this relevant? Doesn't this just indicate that you have a poor join-order somewhere else?

Hmm, actually you might be right. I'll turn this into a draft and discuss it with @rdmarsh2 on what his use-case for it was. Looking at my evaluation log file (which I thought was blowing up due to too many expressions) is really just blowing up due to a bad join in unequalIntegralSsa 🤦. Thanks for the quick comment!