Allow other using alias to be used to the right of the equal sign of the using alias.

[Summpot]

The following writeup is not supported in c# now.

using A = int;
using unsafe B = delegate* unmanaged<A>;

It will report an error.
error CS0246: The type or namespace name 'A' could not be found (are you missing a using directive or an assembly reference?)

As in the example I gave, this also enables named function pointers.

Since using alias will be expanded at compile time, the following problem mentioned in the proposal for function pointers seems to no longer exist.

Part of the problem here is the underlying CLI primitive doesn't have names hence this would be purely a C# invention and require a bit of metadata work to enable. That is doable but is a significant about of work. It essentially requires C# to have a companion to the type def table purely for these names.

[ufcpp]

#3873