Resolves #5210.

Changes:

• This fixes the comments in xstrxform.cpp and xwcsxfrm.cpp: The functions return SIZE_MAX on error, not INT_MAX like their UCRT counterparts.
• In _Wcsxfrm(), the return value is now correctly set to static_cast<size_t>(-1) for one error case, bringing it in line with the other error case. Previously, it was set to INT_MAX, which is potentially a valid length for a sort key.
  • While this is clearly a bug in the code, I couldn't actually find any way to exert this error path. (I was a bit disappointed to find that my fix can't actually be observed to fix anything, but on the other hand, this means that we don't have to worry about any potential compatibility impact.)
• I changed collate::do_transform() to return an empty string in the error case instead of throwing a confusing length_error("string too large").
  • The standard remains completely silent on what collate::do_transform() should do in case of error.
  • The empty string can only really be a valid return value for the empty string, so there isn't any potential for confusion whether the return value represents an actual sort key or an error.
  • Alternatively, we could consciously choose that the function throws an exception. I would prefer this exception to be more appropriate than length_error. Maybe runtime_error?
    • This would mean that the regex implementation would have to defend against this exception, because regex_match and regex_search really shouldn't throw just because the searched input string isn't valid UTF-8.