libstdc++: Fix overconstrained std::string constructor [PR103919]

The C++17 basic_string(const T&, size_t, size_t) constructor is
overconstrained, so it can't be used for a NTBS and a temporary string
gets constructed (potentially allocating memory). There is no
corresponding constructor taking an NTBS, so no need to disambiguate
from it. Accepting an NTBS avoids the temporary (and potential
allocation) and is what the standard requires.

libstdc++-v3/ChangeLog:

	PR libstdc++/103919
	* include/bits/basic_string.h (basic_string(const T&, size_t, size_t)):
	Relax constraints on string_view parameter.
	* include/bits/cow_string.h (basic_string(const T&, size_t, size_t)):
	Likewise.
	* testsuite/21_strings/basic_string/cons/char/103919.cc: New test.