When generating a stub fix-it for a protocol conformance or implementation extension, Swift will now evaluate whether the context allows the declaration of stored properties and, if so, will suggest one. It will also use the `let` keyword instead of `var` if the property has no setter.
* [CSDiagnostics] Emit fix-its to insert requirement stubs in editor mode for missing protocols in context
* [CSDiagnostics] Only include missing requirements in stub fix-it for missing protocols in context
The conforming type may already have declarations that could satisfy a requirement, so we shouldn't include it in the fix-it
