Allow rooting a generic method in RD.XML

[MichalStrehovsky]

Adds support for following syntax:

<Directives xmlns="http://schemas.microsoft.com/netfx/2013/01/metadata">
  <Application>
    <Assembly Name="repro">
      <Type Name="Program">
        <Method Name="SayHello">
          <GenericArgument Name="System.Object, System.Private.CoreLib" />
        </Method>
      </Type>
    </Assembly>
  </Application>
</Directives>

Since this is temporary code, I just twisted the RD.XML syntax a bit so that we can reuse the type string parser we have for custom attribute blobs.

[MichalStrehovsky]

@sergiy-k PTAL

[sergiy-k]

@MichalStrehovsky, thank you! I assume that you checked that this is sufficient to allow rooting the generics method from #4444 (comment) in RD.xml instead of putting it in the code?

[MichalStrehovsky]

@MichalStrehovsky, thank you! I assume that you checked that this is sufficient to allow rooting the generics method from #4444 (comment) in RD.xml instead of putting it in the code?

I haven't actually tried with that, but the syntax should be along the lines of:

<Type Name="SharpYaml.Serialization.Descriptors.DictionaryDescriptor">
  <Method Name="GetGenericEnumerable" Dynamic="Required">
    <GenericArgument Name="System.String, System.Private.CoreLib" />
    <GenericArgument Name="System.Object, System.Private.CoreLib" />
  </Method>
</Type>

It should be possible to express anything with this syntax, unless there's multiple overloads of the method (overload handling would make this unnecessarily complicated so lets cross that bridge if we ever get there).