Initial work to support EnumeratorCancellation.

[bartdesmet]

See:

• https://github.com/dotnet/csharplang/blob/master/meetings/2019/LDM-2019-04-15.md
• https://github.com/dotnet/csharplang/blob/master/meetings/2019/LDM-2019-04-29.md
• Add EnumeratorCancellationAttribute standard#1160

This PR adds the attribute for downlevel platforms, and leverages it for IAsyncEnumerable<T>-based iterators where the Create(Core) pattern using an IAsyncEnumerator<T>-based iterator has been used. The result will be reduction of allocations.

Goal is to ship with HAS_ASYNC_ENUMERABLE_CANCELLATION always set (and hence #if checks removed). Right now, the flag has to be set explicitly because we don't yet have a compiler drop that picks up on this attribute.

When we prepare for the final release in concert with C# 8.0 and .NET Core 3.0 shipping, we'll also revisit the #if USE_ASYNC_ITERATOR checks; we should be able to get rid of a lot of hand-rolled state machines then.

[clairernovotny]

I think we need an #else with a TypeForwardedTo here to unify like the rest of the types.

[codecov]

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[bartdesmet]

This is currently blocked on the lack of support for custom attributes on local functions, see dotnet/csharplang#1888. The LDM decision is to add support for these in order to unblock this; see https://github.com/dotnet/csharplang/blob/master/meetings/2019/LDM-2019-04-29.md.

We realized that attributes are not permitted on local function parameters, which means that you cannot use the EnumeratorCancellation attribute in local function iterators.

Conclusion
Allow attributes on local function parameters and type parameters.

Also note that if we merge this PR, we should consider converting some of these local functions to be static to avoid an additional allocation. Other than the "local" nature of local functions (thus not having to come up with a name for another method), we'd pretty much not be using any of the features of local functions (closure, delegate conversion), so if we really care deeply about optimizing these query operators before we get custom attributes on local functions, we could turn them into non-local functions (with *Core method names).