Fixed and improved serializability analyzers

[acizmarik]

This PR introduces a couple of fixes and improvements to serializability analyzers.

• Fixed bug: nullables were sometimes incorrectly marked as not serializable.
• Fixed bug: certain supported types were incorrectly marked as not serializable (GridViewDataSet<T>, UploadedFilesCollection and BusinessPackDataSet<T>). They do not follow our general rules, however their serialization is well-defined and should not issue warnings.
• Fixed bug: that caused that some user-defined generic reference types were incorrectly marked as not serializable
• New improvement: symbols obtained from a specific Compilation within a specific SemanticModel are no longer cached indefinitely. Instead, we now construct cache for each Compilation. These caches are held using weak references.
• New improvement: sanity check. Serializability analyzers perform a simple sanity check to ensure that everything works before issuing warnings. (This might not be necessary anymore but I decided to leave it there).
• New improvement: serializability warnings now issue property paths. This is more straight-forward when looking for the root of the issue - especially when working with multiple levels of nested classes.

Additionally, DateTimeOffset was added to supported types. Analyzers no longer generate serializability warnings for properties that implement IEnumerable (however element type needs to be still serializable).

[acizmarik]

It seems like Analyzer tests are not run on CI currently