[Proposal] Create new objects from anonymous types

I suggest this general solution to the issue in #576:
Suppose we have this anonymous type:
`Dim a = New With {.x = 1, .y = 2}`
I suggest to allow reusing it like this:
`Dim b = New TypeOf(a) With {.x = 3, .y = 4}`
And in a short form, using a virtual constructor:
`Dim c = New TypeOf(a)(3, 4)`
VB can supstitute `TypeOf(a)` with the actual temp type used for the anonymous type, and convert the constructor to filed assignment statements, in the same order they defined in the type.