Fix PR#31454 - 'basic_string<T>::push_back() crashes if sizeof(T)>sizeof(long...

Fix PR#31454 - 'basic_string<T>::push_back() crashes if sizeof(T)>sizeof(long long)'. We were mishandling the small-string optimization calculations for very large 'characters'. This may be an ABI change (change the size of) strings of very large 'characters', but since they never worked, I'm not too concerned.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@324531 91177308-0d34-0410-b5e6-96231b3b80d8