[GenericSigBuilder] Conformances due to concrete types can be abstract.

When a type parameter is made concrete via an existential type,
conformance requirements on that type parameter will be
abstract. Fixes rdar://problem/30610428.

Author: DougGregor

lib/AST/GenericSignatureBuilder.cpp

@@ -1934,10 +1934,15 @@ bool GenericSignatureBuilder::addSameTypeRequirementToConcrete(
 
       conformances.insert({protocol, *conformance});
 
+      // Abstract conformances are acceptable for existential types.
+      assert(conformance->isConcrete() || Concrete->isExistentialType());
+
       // Update the requirement source now that we know it's concrete.
       // FIXME: Bad concrete source info.
       auto concreteSource = Source->viaConcrete(*this,
-                                                conformance->getConcrete());
+                                                conformance->isConcrete()
+                                                  ? conformance->getConcrete()
+                                                  : nullptr);
       updateRequirementSource(conforms.second, concreteSource);
     }
   }

test/Generics/requirement_inference.swift

@@ -230,3 +230,20 @@ struct X11<T: P12, U: P12> where T.B == U.B.A {
 	// CHECK: Canonical generic signature: <τ_0_0, τ_0_1, τ_1_0 where τ_0_0 : P12, τ_0_1 == X10, τ_0_0.B == X10>
 	func upperSameTypeConstraint<V>(_: V) where U == X10 { }
 }
+
+#if _runtime(_ObjC)
+// rdar://problem/30610428
+@objc protocol P14 { }
+
+class X12<S: AnyObject> {
+  func bar<V>(v: V) where S == P14 {
+  }
+}
+
+@objc protocol P15: P14 { }
+
+class X13<S: P14> {
+  func bar<V>(v: V) where S == P15 {
+  }
+}
+#endif