[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 2172154840d2f6b0e37f89d589de8d0fc83fe061..acf783cc417eae1aceb937b206c0900bb52c7241 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();