Allow pattern-based disposal in foreach

[jcouv]

It seems that foreach only every binds disposal via the interface, and never via pattern-based approach.
I expect that foreach would bind pattern-based Dispose() methods on ref struct enumerators, at least.

Note: in that scenario, the compiler should not produce enumerator as IDisposable logic.

Note: there is a similar issue with pattern-based disposal in await foreach, except that we likely need to lift the ref struct restriction there. (see #32316)

Update (1/16): from LDM today, we've decided that extension methods will not contribute to pattern-based disposal, even if we decide they contribute for binding pattern-based GetEnumerator() in a foreach.

Tagging @agocke

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Per notes (emphasis mine):

When we start recognizing pattern Dispose, do we call it in foreach?

Certainly, it seems very important for the feature. Since a ref struct
cannot implement interfaces, pattern Dispose is the only way to implement
disposal.

There are a number of possible breaking changes:

1. A pattern enumerable type did not implement IDisposable, but had a
   Dispose method. This method would now be call.
2. If there are two Dispose extension methods, that will now produce
   an ambiguity and thus a compilation error.

Conclusion

Let's narrow the pattern-based Dispose change down to ref structs only.
This means every other type will be required to implement IDisposable.

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Umbrella for enhanced-using feature: #28588

[chsienki]

This is implemented and working