[Sema] Implement type checking for the _typeEraser attribute

[hborla]

The rules for @_typeEraser are:

1. The type eraser must be a concrete nominal type
2. The type eraser must not have more restrictive access than the annotated protocol
3. The type eraser must conform to the annotated protocol
4. The type eraser must have an initializer of the form init<T: AnnotatedProtocol>(erasing: T)
   • Other generic requirements are okay as long as the init can always be called with a value of any type conforming to the protocol
   • The init cannot have more restrictive access than the annotated protocol

[hborla]

@swift-ci please smoke test

[hborla]

@swift-ci please smoke test

[xedin]

Looks great!

[hborla]

@swift-ci please smoke test