Mark string_view's constructor from (ptr,len) as noexcept (an extension). Update the tests to check this (and other noexcept bits

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@316456 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/string_view b/include/string_view
index c5cc255..3e112eb 100644
--- a/include/string_view
+++ b/include/string_view
@@ -216,7 +216,7 @@
 	basic_string_view& operator=(const basic_string_view&) _NOEXCEPT = default;
 
 	_LIBCPP_CONSTEXPR _LIBCPP_INLINE_VISIBILITY
-	basic_string_view(const _CharT* __s, size_type __len)
+	basic_string_view(const _CharT* __s, size_type __len) _NOEXCEPT
 		: __data(__s), __size(__len)
 	{
 // #if _LIBCPP_STD_VER > 11