SL-10153: Improve ll_convert_string_to_wide() and its converse.

Instead of returning a wchar_t* and requiring the caller to delete it later,
return a std::basic_string<wchar_t> that's self-cleaning. If the caller wants
a wchar_t*, s/he can call c_str() on the returned string.

Default the code_page parameter to CP_UTF8, since we try to be really
consistent about using UTF-8 encoding for all our internal std::strings.