[WIP] Basic support for non-nullable references

[roji]

This WIP PR adds first support for taking the new C# 8.0 non-nullable references into account in model building. This adds convention logic that interprets non-nullable references in the same way as [Required] - for both regular and navigation properties.

Some notes:

• No scaffolding work done yet, we still have to decide what our scaffolding strategy is for this.
• The ConfigurationSource for non-nullable references is currently Convention, as non-nullability reference nullability seems weaker than [Required] or the fluent API, and should be overridden by them (there's a test for that).
• For now I integrated the nullability support into the existing RequiredPropertyAttributeConvention and RequiredNavigationAttributeConvention (which I renamed to RequiredPropertyConvention and RequiredNavigationConvention respectively). I'm not sure how fine-grained we like our conventions to be, in theory it may be worth separating them in case somebody doesn't want the new nullability support. Also, we can consider removing NavigationAttributeNavigationConvention as it's no longer used by us - although users may still use it for their own custom attributes?
• Some tests are missing (for non-nullable navigation properties) as well as log message changes.
• Weirdly, the tests pass from the cmdline but Reference_nullability_sets_is_nullable_correctly(NullAwareNonNullable) fails in VS2019 - this is a result of seeing a [Nullable] version from before NullableAttribute constructor parameter should be stored in a field roslyn#30143 (VS is somehow stuck behind, not using the latest Roslyn or something)? @bricelam we briefly discussed this, if you feel like taking a quick look to make sure I'm not crazy....

[vslee]

Since the Milestone of the Roslyn PR you referenced is 16.1.P1, it will show up in Visual Studio 2019 Update 1 Preview 1

[roji]

@vslee thanks for the comment. There's still a bit of work before this can get merged, but hopefully that will happen quite soon.

[roji]

Superceded by #15499