Currently, when the string shrink into the SSO buffer, the `__rep_.__s`
member isn't activated before the `traits_type::copy` call
yet, so internal `__builtin_memmove` call writing to the buffer causes
constant evaluation failure. The existing test coverage seems a bit
defective and doesn't cover this case - `shrink_to_fit` is called on the
copy of string after erasure, not the original string object.
This PR reorders the `__set_short_size` call, which starts the lifetime
of the SSO buffer, before the copy operation. Test coverage is achieved
by calling `shrink_to_fit` on the original erased string.
479f992291