Create analyzer to detect calls to SemanticModel.GetDeclaredSymbol() with fields and other invalid syntax

[JoshVarty]

Using SemanticModel.GetDeclaredSymbol() will always return null for certain pieces of syntax. According to the comments this is the case for GlobalStatementSyntax, IncompleteMemberSyntax, and BaseFieldDeclarationSyntax.

In order to guide new people using the API, we should provide an analyzer that checks for invalid calls to GetDeclaredSymbol() and issues a warning. (In particular the field syntax can be confusing for people who don't realize fields consist of multiple variables).

[Evangelink]

Although I like rules helping you to use an API correctly, I think these use cases are rare.