IRGen: Map type into context before asking for class layout.

Asking for the layout of the interface type apparently gives the wrong answer,
causing key paths to access stored properties of generic classes using the wrong
strategy.

Fixes SR-10167 | rdar://problem/49230751.

Author: jckarter

lib/IRGen/GenKeyPath.cpp

@@ -761,8 +761,13 @@ emitKeyPathComponent(IRGenModule &IGM,
       assert(currentBaseTy->getClassOrBoundGenericClass() == propertyBaseDecl);
       loweredBaseTy =
           IGM.getLoweredType(AbstractionPattern::getOpaque(), currentBaseTy);
+      
+      auto loweredBaseContextTy = SILType::getPrimitiveObjectType(
+            GenericEnvironment::mapTypeIntoContext(genericEnv,
+                                                   loweredBaseTy.getASTType())
+              ->getCanonicalType());
 
-      switch (getClassFieldAccess(IGM, loweredBaseTy, property)) {
+      switch (getClassFieldAccess(IGM, loweredBaseContextTy, property)) {
       case FieldAccess::ConstantDirect: {
         // Known constant fixed offset.
         auto offset = tryEmitConstantClassFragilePhysicalMemberOffset(IGM,

test/IRGen/keypaths.sil

@@ -27,6 +27,9 @@ public class C: Hashable {
   public func hash(into hasher: inout Hasher)
   public static func ==(_: C, _: C) -> Bool
 }
+public class GC<T> {
+  public final var x: T
+}
 
 public struct G<T> {
   public var x: T { get set }
@@ -299,6 +302,14 @@ sil_vtable C2 {}
 // CHECK-32-SAME: i32 24 }>
 // CHECK-64-SAME: i32 48 }>
 
+// -- %gcx: GC<T>.x
+// CHECK: [[KP_GCX:@keypath(\..*)?]] = private global <{ {{.*}} }> <{
+// CHECK-SAME: @"generic environment l"
+// CHECK-SAME: @"symbolic 8keypaths2GCCyxG"
+// CHECK-SAME: @"symbolic x"
+//             -- class with runtime-resolved offset, mutable
+// CHECK-SAME: <i32 0x3fffffe>
+
 // CHECK-LABEL: @"generic environment SHRzSHR_r0_l" = linkonce_odr hidden constant
 // CHECK-SAME: i32 8193, i16 2, i8 -128, i8 -128, i32 128
 
@@ -421,6 +432,14 @@ entry:
   return undef : $()
 }
 
+sil @generic_class_stored_final : $@convention(thin) <T> () -> () {
+entry:
+  %gcx = keypath $ReferenceWritableKeyPath<GC<T>, T>, <T> (root $GC<T>; stored_property #GC.x: $T) <T>
+
+  return undef : $()
+}
+
+
 // CHECK-LABEL: define{{( dllexport)?}}{{( protected)?}} swiftcc void @computed_property_generics
 sil @computed_property_generics : $@convention(thin) <T, U> () -> () {
 entry: