Replace ArgumentType with abstract ArgumentTypeInfo in Descriptor

[KathleenDollard]

ArgumentType is problematic because we don't actually know how the ArgumentType is best represented in the Specific layer, and it must be retrieved via the specific layer (as is done with Reflection). The Type type is actually a Reflection construct, invalid in JSON and probably invalid in Roslyn.

The ArgumentTypeInfo class has the goal of having both a general and a specific way to identify the class. In practice, we anticipate Reflection->Runtime types and Roslyn->Source generation without those streams crossing, and JSON is an intellectual exercise for the present. So it seems silly to force the type into a general format and back again - since that format can't be any better than a string name, with associated hassles about how full of a name to use.

There are a few gnarly details still to work out.