libstdc++: Add noexcept-specifier to basic_string_view(It, End)

libstdc++: Add noexcept-specifier to basic_string_view(It, End)

This adds a conditional noexcept to the C++20 constructor. The
std::to_address call cannot throw, so only taking the difference of the
two iterators can throw.

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>
libstdc++-v3/ChangeLog:

	* include/std/string_view (basic_string_view(It, End)): Add
	noexcept-specifier.
	* testsuite/21_strings/basic_string_view/cons/char/range.cc:
	Check noexcept-specifier. Also check construction without CTAD.