[turbofan] Introduce JSCallForwardVarargs operator.

src/x64/interface-descriptors-x64.cc

Index: src/x64/interface-descriptors-x64.cc
diff --git a/src/x64/interface-descriptors-x64.cc b/src/x64/interface-descriptors-x64.cc
index b7750116817f9b74b3387d91b9b6a057d2d327ba..a4834678dcc39f5a1f43815ea6f7c4e05a63613f 100644
--- a/src/x64/interface-descriptors-x64.cc
+++ b/src/x64/interface-descriptors-x64.cc
@@ -175,6 +175,13 @@ void CallTrampolineDescriptor::InitializePlatformSpecific(
   data->InitializePlatformSpecific(arraysize(registers), registers);
 }
 
+void CallForwardVarargsDescriptor::InitializePlatformSpecific(
+    CallInterfaceDescriptorData* data) {
+  // rcx : start index (to support rest parameters)
+  // rdi : the target to call
+  Register registers[] = {rdi, rcx};
+  data->InitializePlatformSpecific(arraysize(registers), registers);
+}
 
 void ConstructStubDescriptor::InitializePlatformSpecific(
     CallInterfaceDescriptorData* data) {