[regexp] parse RegExpUnicodeEscapeSequence according to spec.

src/regexp/regexp-parser.h

Index: src/regexp/regexp-parser.h
diff --git a/src/regexp/regexp-parser.h b/src/regexp/regexp-parser.h
index 85b9354010e67f9bbe6fedfd389c4ce20d7e1208..05dfebab109240676c46c0b53ef80d51beb79188 100644
--- a/src/regexp/regexp-parser.h
+++ b/src/regexp/regexp-parser.h
@@ -102,6 +102,7 @@ class RegExpBuilder : public ZoneObject {
   RegExpBuilder(Zone* zone, bool ignore_case, bool unicode);
   void AddCharacter(uc16 character);
   void AddUnicodeCharacter(uc32 character);
+  void AddEscapedUnicodeCharacter(uc32 character);
   // "Adds" an empty expression. Does nothing except consume a
   // following quantifier
   void AddEmpty();