[libcxx] [test] Fix nodiscard warnings.

[STL_MSFT]

Summary

[libcxx] [test] Fix nodiscard warnings.

MSVC's STL has marked to_bytes/from_bytes as nodiscard.

[EricWF]

There's a macro for this in test_macros.h to make it easier to search for. please use that

[STL_MSFT]

Committed with a follow-up to use the requested macro.

test/std/localization/locales/locale.convenience/conversions/conversions.string/ctor_err_string.pass.cpp

#if TEST_STD_VER > 11
     static_assert(!std::is_convertible<std::string, Myconv>::value, "");
     static_assert( std::is_constructible<Myconv, std::string>::value, "");
 #endif
 #ifndef TEST_HAS_NO_EXCEPTIONS
     {
         Myconv myconv;
         try
         {
-            myconv.to_bytes(L"\xDA83");
+            (void)myconv.to_bytes(L"\xDA83");
             assert(false);
         }
         catch (const std::range_error&)
         {
         }
         try
         {
-            myconv.from_bytes('\xA5');
+            (void)myconv.from_bytes('\xA5');
             assert(false);
         }
         catch (const std::range_error&)
         {
         }
     }
 #endif
     {
         Myconv myconv("byte error");
         std::string bs = myconv.to_bytes(L"\xDA83");
         assert(bs == "byte error");
 #ifndef TEST_HAS_NO_EXCEPTIONS
         try
         {
-            myconv.from_bytes('\xA5');
+            (void)myconv.from_bytes('\xA5');
             assert(false);
         }
         catch (const std::range_error&)
         {
         }
 #endif
     }
     {
         Myconv myconv("byte error", L"wide error");
         std::string bs = myconv.to_bytes(L"\xDA83");
         assert(bs == "byte error");
         std::wstring ws = myconv.from_bytes('\xA5');
         assert(ws == L"wide error");
     }
 }