C++: Recognize C++11 constexpr

Change-Id: Iac8ba58404284f0d90fd5a3640eb9ccd7e98058d
Reviewed-by: Roberto Raggi <roberto.raggi@nokia.com>

src/libs/3rdparty/cplusplus/Parser.cpp

@@ -3413,6 +3413,10 @@ bool Parser::lookAtStorageClassSpecifier() const
     case T_MUTABLE:
     case T_TYPEDEF:
         return true;
+    case T_CONSTEXPR:
+        if (_cxx0xEnabled)
+            return true;
+        // fall-through
     default:
         return false;
     }