IOperation API for declaration expressions

[mavasani]

We need to add a separate IOperation API to represent declaration expressions, currently they get represented as ILocalReferenceExpressions. See #20749 (comment)

Examples:
M(out var x);
var (x, y) = (1, 2);

[jinujoseph]

We think these should be exposed as their own IOperation. However, if this is difficult to do because of how the bound nodes work, then let us know. We could decide that it's just too expensive and we can settle with just having ILocalReferenceExpressions with some sort of way to map from them to the declaration expression.

[jinujoseph]

@333fred can you verify if we did declarationExpression ?

[333fred]

We settled on IsDeclaration on ILocalReferenceExpression and IFieldReferenceExpression. https://github.com/dotnet/roslyn/blob/master/src/Compilers/Core/Portable/Operations/ILocalReferenceExpression.cs#L21-L25

[333fred]

Dupe of #18303.

[jinujoseph]

how about tuples var (x, y) = (1, 2); scenario ?

[333fred]

That was one of the tests I added.