Virtual AttributeType property and signature generic types (#19818)

Virtual AttributeType property and signature generic types (#19818)

* Virtual AttributeType property and signature generic types

https://github.com/dotnet/corefx/issues/31614

1. This will allow Reflection providers the option
to supply the attribute type without building
an entire constructor.

https://github.com/dotnet/corefx/issues/31798

2. This will permit other Reflection providers
to support Type.MakeGenericMethodParameter()
in their implementations.

* More robust argument validation.

* Change parameter name