[Version 9.0] Feature support for covariant return types

[RexJaeschke]

This PR is my formal version of the corresponding MS spec.

The following issues are yet not handled by this PR and need to be addressed:

1. 12.8.12.4 Indexer access: The MS spec states, “A similar change should be made in §12.8.12.4 Indexer access.” However, no proposed edits were given.
2. 15.7.6 Virtual, sealed, override, and abstract accessors: The MS spec states, “We similarly permit override properties and indexers in interfaces as specified for classes in §15.7.6 Virtual, sealed, override, and abstract accessors.” However, no proposed edits were given.
3. 19 Interfaces: The MS spec states, “We will need a rule that an explicit interface implementation must declare a return type no less derived than the return type declared in any override in its base interfaces.” However, no proposed edits were given.
4. 19.6.5 interface mapping: The MS spec states, “This is technically a breaking change, … Due to this breaking change, we might consider not supporting covariant return types on implicit implementations.” We need to check if it’s supported.