Fixes for EXT-4689 (Long-word chat spam cripples fps and/or disconnects client)

Removed some unnecessary string copies in LLTextBase and LLNormalTextSegment by changing getWText() to return const LLWString& (instead of LLWString), and storing the result in a const LLWString& instead of an LLWString in places where the string doesn't need to be modified.

Made LLTextViewModel::getDisplay() also return const LLWString& instead of LLWString.

Removed a couple of unused local string variables in LLTextBase (the compiler flagged these after converting the local copies to const LLWString&).