Adding an analyzer to the CodeAnalysis analyzers package that issues an error if someone tries to implement an interface marked with an attribute.

Adding a internal type (System.Runtime.CompilerServices.InternalImplementationOnlyAttribute) to CodeAnalysis.dll and applying it to ISymbol so that we can prevent folks from implementing ISymbol (in order for us to be able to make changes to it in the future). Chose the System namespace because the LDM wants this to be a compiler feature in the future. The analyzer just checks by name and so the type can be internal and if a different dll needs it, it can define the attribute by itself. (changeset 1394725)