blocks:

• SR-257 Resilient value type support -- umbrella bug

Issue Description:

Define this in file a.swift:

struct S {
var x: Int
}

And this in b.swift:

protocol P {
var x: Int { get }
}
extension S : P {}

When built without whole-module optimization, Sema lazily creates the accessor for S.x but SILGen never emits this.

This is an easy fix, just requires tightening up semantics of ASTContext.ExternalDefinitions and related code.

Probably also the accessor needs to be emitted with shared linkage even if the struct is public, to avoid clashes if it is re-emitted from another module. This is OK since the struct is fixed-layout.