Request handlers defined in partial classes cannot be resolved correctly

[andreesteve]

When a handler is defined in multiple partial classes, we fail to correctly parse the handler metadata.
Happens in this particular line:

crthandyman/Handyman/DocumentAnalyzers/RequestHandlerAnalyzer.cs

Line 102 in b07f7b3

declaredSupportedRequestTypes = Enumerable.Empty<ITypeSymbol>();