stringProtoFuncReplace converts 8 bit strings to 16 bit during replacement

stringProtoFuncReplace converts 8 bit strings to 16 bit during replacement
https://bugs.webkit.org/show_bug.cgi?id=76651

Reviewed by Geoffrey Garen.

Made local function substituteBackreferencesSlow a template function
based on character width.  Cleaned up getCharacters() in both UString
and StringImpl.  Changed getCharacters<UChar> to up convert an 8 bit
string to 16 bits if necessary.

* runtime/StringPrototype.cpp:
(JSC::substituteBackreferencesSlow):
(JSC::substituteBackreferences):
* runtime/UString.h:
(JSC::LChar):
(JSC::UChar):
* wtf/text/StringImpl.h:
(WTF::UChar):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@106257 268f45cc-cd09-0410-ab3c-d52691b4dbfc