Add tests to ensure that we weak-link associated conformances introduced later

Author: DougGregor

test/IRGen/weak_import_associated_conformance_descriptor.swift

@@ -19,6 +19,13 @@ public protocol StrongProtoWithWeakLinkedAssoc {
   @_weakLinked associatedtype Assoc: P
 }
 
+public struct LibConformsToP: P { }
+
+public protocol StrongProtoWithNewAssoc {
+  @available(macOS 10.50, *)
+  associatedtype NewerAssoc: P = LibConformsToP
+}
+
 @available(macOS 10.50, *)
 public protocol WeakProtoWithAssoc {
   associatedtype Assoc: P
@@ -40,11 +47,18 @@ struct ConformsToStrongProtoWithAssoc: StrongProtoWithAssoc {
   typealias Assoc = ConformsToP
 }
 
+
 // CHECK: @"$s7Library30StrongProtoWithWeakLinkedAssocP0G0AC_AA1PTn" = extern_weak global %swift.protocol_requirement, align 4
 struct ConformsToStrongProtoWithWeakLinkedAssoc: StrongProtoWithWeakLinkedAssoc {
   typealias Assoc = ConformsToP
 }
 
+// CHECK: @"$s7Library23StrongProtoWithNewAssocP05NewerF0AC_AA1PTn" = extern_weak global %swift.protocol_requirement, align 4
+// CHECK: @"$s10NewerAssoc7Library018StrongProtoWithNewB0PTl" = extern_weak global %swift.protocol_requirement, align 4
+struct ConformsToStrongProtoWithNewAssoc: StrongProtoWithNewAssoc {
+  typealias NewAssoc = ConformsToP
+}
+
 // CHECK: @"$s7Library18WeakProtoWithAssocP0E0AC_AA1PTn" = extern_weak global %swift.protocol_requirement, align 4
 @available(macOS 10.50, *)
 struct ConformsToWeakProtoWithAssoc: WeakProtoWithAssoc {