[c#] support for global using directives in CSharpProgramSummary

[xavierpinho]

Adds support for handling global using directives. These are distinguished both during summarisation and in CSharpProgramSummary, so that findGlobalTypes accounts for them.

[DavidBakerEffendi]

So I believe the original design choice was to assign globals to the global namespace. Does this not work for your use-case?

[xavierpinho]

Good question. From what I've seen, this Global namespace is being used to store top-level user-defined types found during pre-analysis. For using global XXX, we want to have XXX available in typesInScope but XXX is to be found in the program summary's NamespaceToTypeMap. Perhaps the best patch would be to add globalImports directly to typesInScope -- currently it's set to this findGlobalTypes. What do you reckon?

To illustrate, the newer patch I'm proposing would look something like (in CSharpScope.scala):

  override val typesInScope: mutable.Set[CSharpType] = {
    val globalTypes = summary.findGlobalTypes
    val globallyImportedTypes = summary.globalImports.flatMap(summary.namespaceToType.getOrElse(_, Set.empty))
    val result = mutable.Set.empty[CSharpType]
    result.addAll(globalTypes)
    result.addAll(globallyImportedTypes)
    result
  }

EDIT: See 3e016b4