`GetPreprocessingSymbolInfo` returns info when inside `#pragma warning` directive

During #66425 we discovered that `CSharpSemanticModel.GetPreprocessingSymbolInfo(IdentifierNameSyntax)` returns valid preprocessing info when invoked on an `IdentifierNameSyntax` inside a `#pragma warning` directive, where the identifier is not a preprocessing symbol.

Upon request of @CyrusNajmabadi