[csa] Migrate String.prototype.concat to TurboFan builtin.

src/bootstrapper.cc

Index: src/bootstrapper.cc
diff --git a/src/bootstrapper.cc b/src/bootstrapper.cc
index 65208d7f3241b01d084c4f3ed6b8dc48c390da4c..81e01224f768b926bdc18d06ca492f009393e17c 100644
--- a/src/bootstrapper.cc
+++ b/src/bootstrapper.cc
@@ -1688,6 +1688,8 @@ void Genesis::InitializeGlobal(Handle<JSGlobalObject> global_object,
                           1, true);
     SimpleInstallFunction(prototype, "charCodeAt",
                           Builtins::kStringPrototypeCharCodeAt, 1, true);
+    SimpleInstallFunction(prototype, "concat", Builtins::kStringPrototypeConcat,
+                          1, false);
     SimpleInstallFunction(prototype, "endsWith",
                           Builtins::kStringPrototypeEndsWith, 1, false);
     SimpleInstallFunction(prototype, "includes",