Reviewed by Darin Adler.

        UTF-16 code points compare() for String objects
        https://bugs.webkit.org/show_bug.cgi?id=39701

        Moving compare() implementation from UString to StringImpl for it to be shared
        with String. Adding overloaded free functions codePointCompare() in StringImpl
        and WTFString. Renaming function compare in UString to codePointCompare to be
        consistent.

        * runtime/JSArray.cpp:
        (JSC::compareByStringPairForQSort):
        * runtime/UString.cpp:
        * runtime/UString.h:
        (JSC::codePointCompare):
        * wtf/text/StringImpl.cpp:
        (WebCore::codePointCompare):
        * wtf/text/StringImpl.h:
        * wtf/text/WTFString.cpp:
        (WebCore::codePointCompare):
        * wtf/text/WTFString.h:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@60332 268f45cc-cd09-0410-ab3c-d52691b4dbfc