Fix -Wsign-compare warnings in parser, scanner, regexp, runtime.

src/builtins/builtins-string.cc

Index: src/builtins/builtins-string.cc
diff --git a/src/builtins/builtins-string.cc b/src/builtins/builtins-string.cc
index f210b8d9b4e666001a000d132a77c4ea35e0b096..9fda26831e93de497b18da0da94f6ac55c9f7250 100644
--- a/src/builtins/builtins-string.cc
+++ b/src/builtins/builtins-string.cc
@@ -601,7 +601,7 @@ BUILTIN(StringFromCodePoint) {
   List<uc16> two_byte_buffer(length - index);
 
   while (true) {
-    if (code <= unibrow::Utf16::kMaxNonSurrogateCharCode) {
+    if (code <= static_cast<uc32>(unibrow::Utf16::kMaxNonSurrogateCharCode)) {
       two_byte_buffer.Add(code);
     } else {
       two_byte_buffer.Add(unibrow::Utf16::LeadSurrogate(code));