FindAllReferences should handle `Dispose` referenced in `using` and `foreach` statements

See comment in `ComputeImplicitReferenceConflictsAsync`: ` // TODO: handle Dispose for using statement and Add methods for collection initializers.`

Note, if this done at the same time or after C# 8.0, FAR should also account for `DisposeAsync` in async-foreach and async-dispose statements.

Relates to https://github.com/dotnet/roslyn/pull/28230