C# 7.3: auto-implemented property field-targeted attributes

[BillWagner]

This feature would enable this syntax:

[field: SomeThingAboutFieldAttribute]
public int SomeProperty { get; set; }

The attribute SomeThingAboutFieldAttribute would be applied to the compiler generated backing field for SomeProperty

See dotnet/csharplang#42

This likely does not introduce any new articles. Instead, it will be looking at where we discuss Attributes and updates any locations where we say this isn't allowed.

[BillWagner]

The following files will need to be updated for this feature:

• docs/csharp/properties.md
• docs/csharp/programming-guide/concepts/attributes/attributeusage.md
• docs/csharp/programming-guide/concepts/attributes/index.md
• docs/csharp/programming-guide/concepts/serialization/walkthrough-persisting-an-object-in-visual-studio.md
• docs/csharp/programming-guide/concepts/serialization/index.md

[BillWagner]

The updated proposal is here

[BillWagner]

This will get completed in Sprint 135.

Current status: All the files listed in the comment above have been reviewed, and updated. All the samples have been migrated to the Samples repo.

Remaining work:

• Update properties discussion to include field based attributes on auto-implemented properties.
• Update Serialization sample to include auto-implemented properties that aren't serialized.