libstdc++: Optimize std::string::assign(Iter, Iter) [PR110945]

libstdc++: Optimize std::string::assign(Iter, Iter) [PR110945]

Calling string::assign(Iter, Iter) with "foreign" iterators (not the
string's own iterator or pointer types) currently constructs a temporary
string and then calls replace to copy the characters from it. That means
we copy from the iterators twice, and if the replace operation has to
grow the string then we also allocate twice.

By using *this = basic_string(first, last, get_allocator()) we only
perform a single allocation+copy and then do a cheap move assignment
instead of a second copy (and possible allocation). But that alternative
has to be done conditionally, so that we don't pessimize the native
iterator case (the string's own iterator and pointer types) which
currently select efficient overloads of replace which will not allocate
at all if the string already has sufficient capacity. For C++20 we can
extend that efficient case to work for any contiguous iterator with the
right value type, not just for the string's native iterators.

So the change is to inline the code that decides whether to work in
place or to allocate+copy (instead of deciding that via overload
resolution for replace), and for the allocate+copy case do a move
assignment instead of another call to replace.

For C++98 there is no change, as we can't do an efficient move
assignment anyway, so keep the current code.

We can also simplify assign(initializer_list<CharT>) because the backing
array for an initializer_list is always disjunct with *this, so most of
the code in _M_replace is not needed.

libstdc++-v3/ChangeLog:

	PR libstdc++/110945
	* include/bits/basic_string.h (basic_string::assign(Iter, Iter)):
	Dispatch to _M_replace or move assignment from a temporary,
	based on the iterator type.