[libc++][test] <=> now has a feature-test macro

...which `test/support/test_macros.h` can use to detect compiler support.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@374722 91177308-0d34-0410-b5e6-96231b3b80d8

test/support/test_macros.h

@@ -223,9 +223,8 @@
 #define TEST_SAFE_STATIC
 #endif
 
-// FIXME: Fix this feature check when either (A) a compiler provides a complete
-// implementation, or (b) a feature check macro is specified
-#if !defined(_MSC_VER) || defined(__clang__) || _MSC_VER < 1920 || _MSVC_LANG <= 201703L
+#if !defined(__cpp_impl_three_way_comparison) \
+    && (!defined(_MSC_VER) || defined(__clang__) || _MSC_VER < 1920 || _MSVC_LANG <= 201703L)
 #define TEST_HAS_NO_SPACESHIP_OPERATOR
 #endif