Explore increasing max JSString::m_length to UINT_MAX.

Explore increasing max JSString::m_length to UINT_MAX.
https://bugs.webkit.org/show_bug.cgi?id=163955
<rdar://problem/32001499>

Reviewed by JF Bastien.

This can cause us to release assert on some code paths. I don't
see a reason to maintain this restriction.

* runtime/JSString.h:
(JSC::JSString::length const):
(JSC::JSString::setLength):
(JSC::JSString::isValidLength): Deleted.
* runtime/JSStringBuilder.h:
(JSC::jsMakeNontrivialString):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@221192 268f45cc-cd09-0410-ab3c-d52691b4dbfc