A few places in bits/basic_string.h use `traits_type::copy` to copy
`__str.length() + 1` bytes.

Despite the knowledge that `__str.length()` is not greater than 15 the
compiler emits (and sometimes inlines) a `memcpy` call. That results
in a quite big set of instructions https://godbolt.org/z/j35MMfxzq

Replacing `__str.length() + 1` with `_S_local_capacity + 1` explicitly
forces the compiler to copy the whole `__str._M_local_buf`. As a
result the assembly becomes almost 5 times shorter and without any
function calls or multiple conditional jumps
https://godbolt.org/z/bfq8bxra9

This patch always copies `_S_local_capacity + 1` if working with
`std::char_traits<char>`.

PR libstdc++/112682:
        * include/bits/basic_string.h: Optimize string moves.


P.S.: still not sure that this optimization is not an UB or fine for
libstdc++. However, the assembly looks much better with it.

-- 
Best regards,
Antony Polukhin