MarkReadonlyFieldsRule throws exception on roslyn-analyzers repo

Here is the exception:

MarkReadonlyFieldsRule exception.txt

@michaelcfanning, @srivatsn FYI, the first of the class of failures we discussed.

We are running a global semantic rule whose SupportsLanguage method returns true only for C# on a VB file (it crashes on DoNotCatchCorruptedStateExceptionsAnalyzer.vb).

The method FormattingEngineImplementation.RunSyntaxPass runs only the rules appropriate to the specified language. But RunLocalSemanticPass and RunGlobalSemanticPass don't filter on language (they don't take a languageName argument at all).

This is kind of understandable -- the intent seems to have been that semantic rules are language independent. But not all of them are. The class WritableFieldScanner used by the failing rule is derived from CSharpSyntaxWalker.