Refactor RequiredAttributeParser and decouple it from the builder

[ajaybhargavb]

From aspnet/Razor#1054 (comment)

@rynowak @NTaylorMullen

[rynowak]

There should be a simple set of static methods that can parse our 'required attribute' syntax. The builder should call into that, and then use the results to 'build' the descriptor. Right now this code is coupled to the builder and it shouldn't be.