[Proposal] Ease the pain of using non-generic (object) collections in For Each

I have this code:
```
Protected Function GetAlwaysVisibleItems() As IEnumerable(Of CompletionItem)
Return From elm As XmlSchemaElement In HtmlSchema.Elements.Values
Select GetItem(elm.Name)
End Function
```

And I get the error:
> Option Strict On disallows implicit conversions from 'Object' to 'XmlSchemaElement'.

I have two solutions:
1- Use `Option Strict Off` which I don't want to do for all the file.
2- Use explicit cast
```VB.NET
Protected Function GetAlwaysVisibleItems() As IEnumerable(Of CompletionItem)
Return From elm In HtmlSchema.Elements.Values
Select GetItem(CType(elm, XmlSchemaElement).Name)
End Function
```

which is a long of unnecessary code.

I need two things here:
1. A way to make VB accept the implicit casting in for each. If VB will not do that by default , maybe we can use `[]` around the type as an indication.
```VB.NET
Protected Function GetAlwaysVisibleItems() As IEnumerable(Of CompletionItem)
Return From elm As [XmlSchemaElement] In HtmlSchema.Elements.Values
Select GetItem(elm.Name)
End Function
```

this syntax is accepted today as the `[]` are used for keyword dis-ambiguity. If this is not acceptable for some consideration, we can instead use it around `As type`:
```VB.NET
Protected Function GetAlwaysVisibleItems() As IEnumerable(Of CompletionItem)
Return From elm [As XmlSchemaElement] In HtmlSchema.Elements.Values
Select GetItem(elm.Name)
End Function
```

2. I want VB to infer the non-generic collection's element type from its indexer. I know that may break existing code, so what I am asking if to auto add `[As XmlSchemaElement]` in the for each / from , or offer it as the suggested type in auto complete.