[CSApply] Optional chaining forces key path component to be r-value

Key path components with optional chaining unlike other places
don't propagate l-value through optionals.

Resolves: rdar://123638701

test/SILGen/keypaths.swift

@@ -652,3 +652,23 @@ struct Test {
   var codingPath: [any CodingKey] { codingStack.map(\.key) }
   // CHECK: keypath $KeyPath<CodingStackEntry, URICoderCodingKey>, (root $CodingStackEntry; stored_property #CodingStackEntry.key : $URICoderCodingKey)
 }
+
+// rdar://123638701 - Make sure that optional chaining forces loads.
+func test_optional_chaining_with_function_conversion() {
+  class Storage {}
+
+  class Elements {
+    var db: Storage = Storage()
+  }
+
+  class Source {
+    var elements: Elements? = nil
+  }
+
+  func test(data: [Source]) {
+    // CHECK: {{.*}} = keypath $KeyPath<Source, Optional<Storage>>
+    _ = data.compactMap(\.elements?.db)
+    // CHECK: {{.*}} = keypath $KeyPath<Source, Storage>
+    _ = data.compactMap(\.elements!.db)
+  }
+}