Fix LifetimeDependence feature guard for variable accessors.

Guard variable declarations by `#if $LifetimeDependence` if they have a
~Escapable type. Accessing the declaration implies a lifetime dependence.

lib/AST/FeatureSet.cpp

@@ -254,14 +254,16 @@ static bool usesFeatureLifetimeDependence(Decl *decl) {
   if (decl->getAttrs().hasAttribute<LifetimeAttr>()) {
     return true;
   }
-  auto *afd = dyn_cast<AbstractFunctionDecl>(decl);
-  if (!afd) {
-    return false;
-  }
+  if (auto *afd = dyn_cast<AbstractFunctionDecl>(decl)) {
     return afd->getInterfaceType()
       ->getAs<AnyFunctionType>()
       ->hasLifetimeDependencies();
   }
+  if (auto *varDecl = dyn_cast<VarDecl>(decl)) {
+    return !varDecl->getTypeInContext()->isEscapable();
+  }
+  return false;
+}
 
 UNINTERESTING_FEATURE(DynamicActorIsolation)
 UNINTERESTING_FEATURE(NonfrozenEnumExhaustivity)

test/ModuleInterface/Inputs/lifetime_dependence.swift

@@ -54,3 +54,24 @@ public func deriveThisOrThat(_ this: consuming BufferView, _ that: consuming Buf
   return BufferView(that._ptr, that._count)
 }
 
+@_unsafeNonescapableResult
+@_transparent
+@lifetime(borrow source)
+internal func _overrideLifetime<T: ~Copyable & ~Escapable, U: ~Copyable & ~Escapable>(
+  _ dependent: consuming T, borrowing source: borrowing U) -> T {
+  dependent
+}
+
+public struct Container {
+  var buffer: UnsafeRawBufferPointer
+  var object: AnyObject
+}
+
+extension Container {
+  public var storage: BufferView {
+    get {
+      let view = BufferView(buffer, 1)
+      return _overrideLifetime(view, borrowing: self)
+    }
+  }
+}

test/ModuleInterface/lifetime_dependence_test.swift

@@ -35,3 +35,9 @@ import lifetime_dependence
 
 // CHECK: @lifetime(this, that)
 // CHECK-NEXT: @inlinable public func deriveThisOrThat(_ this: consuming lifetime_dependence.BufferView, _ that: consuming lifetime_dependence.BufferView) -> lifetime_dependence.BufferView {
+
+// Check that an implicitly dependent variable accessor is guarded by LifetimeDependence.
+//
+// CHECK: extension lifetime_dependence.Container {
+// CHECK-NEXT: #if compiler(>=5.3) && $NonescapableTypes && $LifetimeDependence
+// CHECK-NEXT:   public var storage: lifetime_dependence.BufferView {