IRGen: Use base-relative offsets to identify methods in fragile subclasses of resilient bases in key paths.

The resilient methods will all be keyed by their dispatch thunks, so for methods of local subclasses, we can use the offsets relative to the dynamic base as identifiers without having to adjust for that dynamic base.

test/stdlib/KeyPathMultiModule.swift

@@ -33,9 +33,13 @@ class LocalSub: ResilientSub {
   }
 
   final var storedD: String = "zim"
+  final var storedE: String = "zang"
 
-  var localSub: String {
-    get { return "zang" }
+  var localSubA: String {
+    get { return "zung" }
+  }
+  var localSubB: String {
+    get { return "zipiti" }
   }
 }
 
@@ -174,13 +178,25 @@ keyPathMultiModule.test("identity across multiple modules") {
                           \ResilientSub.subRO)
 
     // Ensure that we can instantiate key paths on a local subclass of a
-    // resilient class.
-    expectEqual(\LocalSub.storedA, \LocalSub.storedA)
-    expectEqual(\LocalSub.storedB, \LocalSub.storedB)
-    expectEqual(\LocalSub.storedC, \LocalSub.storedC)
-    //TODO expectEqual(\LocalSub.storedD, \LocalSub.storedD)
-    expectEqual(\LocalSub.virtual, \LocalSub.virtual)
-    //TODO expectEqual(\LocalSub.localSub, \LocalSub.localSub)
+    // resilient class, and that they have distinct values.
+    let kps: [PartialKeyPath<LocalSub>] = [
+      \LocalSub.storedA,
+      \LocalSub.storedB,
+      \LocalSub.storedC,
+      \LocalSub.storedD,
+      \LocalSub.storedE,
+      \LocalSub.virtual,
+      \LocalSub.sub,
+      \LocalSub.localSubA,
+      \LocalSub.localSubB,
+    ]
+
+    for i in kps.indices {
+      for j in kps.indices {
+        if i == j { continue }
+        expectNotEqual(kps[i], kps[j])
+      }
+    }
 
     func testInGenericContext2<W: ResilientSubProto>(_: W.Type) {
       expectEqualWithHashes(ResilientRootProto_root_keypath(W.self),