[AutoDiff] Rename @differentiating to @derivative(of:).

[dan-zheng]

Rename @differentiating to @derivative(of:). @derivative(of:) more
clearly evokes derivative registration; the syntax is otherwise unchanged.

Deprecate @differentiating, to be removed in the next release.

Discussed here: #28321 (comment).

Partially resolves TF-999.
TF-1000 tracks updating all @differentiating usages across repositories.

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Confirmed via grep -nr differentiating include lib stdlib test that all
remaining occurrences are intended.

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Example:

extension Float {
    @derivative(of: +)
    static func vjpAdd(lhs: Self, rhs: Self) -> (value: Self, pullback: (Self) -> (Self, Self)) {
       (lhs + rhs, { v in (v, v) })
    }
}

[dan-zheng]

Note: I duplicated parseDifferentiatingAttribute rather than adding a flag to parseDerivativeAttribute to be future-facing. It's easier to remove duplicated code.

I avoided unnecessary/duplicate code in other places (e.g. not defining a new DifferentiatingAttr class).

[dan-zheng]

One topic to discuss is whether to show @derivative(of:) or @derivative in user-facing messages.
I chose @derivative(of:), following the precedent of dynamicReplacement(for:).

Code comments mostly use @derivative without the label.

[rxwei]

@dynamicReplacement(for:) doesn't take additional arguments like we do, but @derivative(of:wrt:) is not good either. This works for me.

[dan-zheng]

@swift-ci Please test tensorflow

[rxwei]

I'll merge this in order to get SR-11849 rolling.