[libc++] Remove noexcept specifier from operator""s

For some reason `operator""s(const char8_t*, size_t)` was marked `noexcept`. Remove it and add regression tests.

Reviewed By: ldionne, huixie90, #libc

Spies: libcxx-commits

Differential Revision: https://reviews.llvm.org/D132340

libcxx/include/string

@@ -4736,7 +4736,7 @@ inline namespace literals
 
 #ifndef _LIBCPP_HAS_NO_CHAR8_T
     inline _LIBCPP_HIDE_FROM_ABI constexpr
-    basic_string<char8_t> operator "" s(const char8_t *__str, size_t __len) _NOEXCEPT
+    basic_string<char8_t> operator "" s(const char8_t *__str, size_t __len)
     {
         return basic_string<char8_t> (__str, __len);
     }

libcxx/test/std/strings/basic.string.literals/noexcept.compile.pass.cpp

@@ -0,0 +1,23 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11
+
+#include <string>
+
+#include "test_macros.h"
+
+static_assert(!noexcept(std::operator""s(std::declval<const char*>(), std::declval<int>())), "");
+#ifndef TEST_HAS_NO_CHAR8_T
+static_assert(!noexcept(std::operator""s(std::declval<const char8_t*>(), std::declval<int>())), "");
+#endif
+static_assert(!noexcept(std::operator""s(std::declval<const char16_t*>(), std::declval<int>())), "");
+static_assert(!noexcept(std::operator""s(std::declval<const char32_t*>(), std::declval<int>())), "");
+#ifndef TEST_HAS_NO_WIDE_CHARACTERS
+static_assert(!noexcept(std::operator""s(std::declval<const wchar_t*>(), std::declval<int>())), "");
+#endif