[turbofan] Lower StringCharCodeAt to a dedicated builtin.

src/interface-descriptors.h

Index: src/interface-descriptors.h
diff --git a/src/interface-descriptors.h b/src/interface-descriptors.h
index 7a929269e1c611603dbe2a08480f273b2cc30176..3fbc8d81e64fd7482cf85e1d992cd2b5e5379aa9 100644
--- a/src/interface-descriptors.h
+++ b/src/interface-descriptors.h
@@ -74,6 +74,7 @@ class PlatformInterfaceDescriptor;
   V(CountOp)                              \
   V(StringAdd)                            \
   V(StringCharAt)                         \
+  V(StringCharCodeAt)                     \
   V(StringCompare)                        \
   V(SubString)                            \
   V(Keyed)                                \
@@ -697,6 +698,13 @@ class StringCharAtDescriptor final : public CallInterfaceDescriptor {
                                                CallInterfaceDescriptor)
 };
 
+class StringCharCodeAtDescriptor final : public CallInterfaceDescriptor {
+ public:
+  DEFINE_PARAMETERS(kReceiver, kPosition)
+  DECLARE_DESCRIPTOR_WITH_CUSTOM_FUNCTION_TYPE(StringCharCodeAtDescriptor,
+                                               CallInterfaceDescriptor)
+};
+
 class StringCompareDescriptor : public CallInterfaceDescriptor {
  public:
   DEFINE_PARAMETERS(kLeft, kRight)