libstdc++: Add P1206R7 from_range members to std::string [PR111055]

libstdc++: Add P1206R7 from_range members to std::string [PR111055]

This is the last piece of P1206R7, adding new members to
std::basic_string.

libstdc++-v3/ChangeLog:

	PR libstdc++/111055
	* include/bits/basic_string.h (_S_copy_range): New function.
	(basic_string(from_range_t, R%%, const Alloc&)): New
	constructor.
	(append_range, assign_range, insert_range, replace_with_range):
	New functions.
	* include/bits/cow_string.h: Likewise.
	* testsuite/21_strings/basic_string/cons/from_range.cc: New
	test.
	* testsuite/21_strings/basic_string/modifiers/append/append_range.cc:
	New test.
	* testsuite/21_strings/basic_string/modifiers/assign/assign_range.cc:
	New test.
	* testsuite/21_strings/basic_string/modifiers/insert/insert_range.cc:
	New test.
	* testsuite/21_strings/basic_string/modifiers/replace/replace_with_range.cc:
	New test.

Co-authored-by: Tomasz Kamiński <tkaminsk@redhat.com>