The SSO basic_string has a non-standard insert(iterator, initializer_list)
overload, from a C++0x draft. This adds the correct overload, while also
preserving the old one so that the old symbol is still exported from the
library.

The COW basic_string doesn't have any of the C++11 changes to the insert
overloads (they all still have non-const iterator parameters and the
ones that should return an iterator still return void). This doesn't
make any change to the COW string.

	PR libstdc++/83328
	* acinclude.m4 (libtool_VERSION): Bump to 6:26:0.
	* config/abi/pre/gnu.ver: Add GLIBCXX_3.4.26 and export new symbol.
	* configure: Regenerate.
	* include/bits/basic_string.h [_GLIBCXX_USE_CXX11_ABI]
	(basic_string::insert(const_iterator, initializer_list<C>)): Add.
	[_GLIBCXX_USE_CXX11_ABI && !_GLIBCXX_DEFINING_STRING_INSTANTIATIONS]
	(basic_string::insert(iterator, initializer_list<C>)): Suppress
	definition.
	* include/debug/string (basic_string::insert(iterator, C)): Change
	first parameter to const_iterator.
	(basic_string::insert(iterator, size_type, C)): Likewise. Change
	return type to iterator.
	(basic_string::insert(iterator, InputIterator, InputIterator)):
	Likewise.
	(basic_string::insert(iterator, initializer_list<C>)): Change first
	parameter to const_iterator and return type to iterator.
	* src/c++11/string-inst.cc: Extend comment.
	* testsuite/21_strings/basic_string/modifiers/insert/char/83328.cc:
	New.
	* testsuite/21_strings/basic_string/modifiers/insert/wchar_t/83328.cc:
	New.
	* testsuite/util/testsuite_abi.cc: Add new symbol version.

From-SVN: r261866