[ES6] Support subclassing the String builtin object

[ES6] Support subclassing the String builtin object
https://bugs.webkit.org/show_bug.cgi?id=153068

Reviewed by Michael Saboff.

This patch adds subclassing of strings. Also, this patch fixes a bug where we could have
the wrong indexing type for builtins constructed without storage.

* runtime/PrototypeMap.cpp:
(JSC::PrototypeMap::emptyStructureForPrototypeFromBaseStructure):
* runtime/StringConstructor.cpp:
(JSC::constructWithStringConstructor):
* tests/stress/class-subclassing-string.js: Added.
(test):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@194998 268f45cc-cd09-0410-ab3c-d52691b4dbfc