Change raw string length calculation method (#3772)

New method uses length of source to calculate raw string length.

Also bug with template literal was fixed. Template object should have
indexed properties enumerable.

JerryScript-DCO-1.0-Signed-off-by: Rafal Walczyna r.walczyna@samsung.com

tests/jerry/es2015/tagged-template-literal.js

@@ -126,3 +126,14 @@ assert (String.raw`Hi\n${2+3}!` === "Hi\\n5!");
   var localNew = new getStr();
   assert(chainedCall === getStr() && chainedCall === localNew);
 })();
+
+var templateObject;
+
+(function(p) {
+  templateObject = p;
+})`str`;
+
+var desc = Object.getOwnPropertyDescriptor(templateObject, '0');
+assert(desc.writable === false);
+assert(desc.enumerable === true);
+assert(desc.configurable === false);