Implement non-concepts fallback for coroutine_handle operator<=> (#1249)

* Implement non-concepts fallback for coroutine_handle operator<=>

... to allow EDG to compile `<coroutine>`. [This is a dual of MSVC-PR-271029.]

* Reorder feature-tests in `<experimental/generator>`

... so it won't notice that Edge defines both `__cpp_impl_coroutine` and `__cpp_coroutines` in C++20 mode.

Also don't test `VSO_0971246_legacy_await_headers` with `/BE /await`; `/await` is meaningless for Edge.

Author: CaseyCarter

stl/inc/coroutine

@@ -126,7 +126,11 @@ _NODISCARD constexpr bool operator==(const coroutine_handle<> _Left, const corou
 
 _NODISCARD constexpr strong_ordering operator<=>(
     const coroutine_handle<> _Left, const coroutine_handle<> _Right) noexcept {
-    return compare_three_way()(_Left.address(), _Right.address());
+#ifdef __cpp_lib_concepts
+    return compare_three_way{}(_Left.address(), _Right.address());
+#else // ^^^ no workaround / workaround vvv
+    return _Left.address() <=> _Right.address();
+#endif // __cpp_lib_concepts
 }
 
 template <class _Promise>

stl/inc/experimental/generator

@@ -9,19 +9,19 @@
 #include <yvals_core.h>
 #if _STL_COMPILER_PREPROCESSOR
 
-#if !defined(__cpp_coroutines) && !defined(__cpp_impl_coroutine)
-#error <experimental/generator> requires /std:c++latest or /await compiler options
-#endif // !defined(__cpp_coroutines) && !defined(__cpp_impl_coroutine)
-
 #ifdef _CPPUNWIND
 #include <exception>
 #endif
 #include <memory>
-#ifdef __cpp_coroutines
-#include <experimental/resumable>
-#else // __cpp_coroutines
+
+#if defined(__cpp_impl_coroutine)
 #include <coroutine>
-#endif // __cpp_coroutines
+#elif defined(__cpp_coroutines)
+#include <experimental/resumable>
+#else // ^^^ legacy coroutines / no coroutine support vvv
+#error <experimental/generator> requires /std:c++latest or /await compiler options
+#endif // ^^^ no coroutine support ^^^
+
 
 #pragma pack(push, _CRT_PACKING)
 #pragma warning(push, _STL_WARNING_LEVEL)

tests/std/tests/VSO_0971246_legacy_await_headers/env.lst

@@ -7,5 +7,4 @@ PM_CL="/EHsc /MT /std:c++latest /permissive-"
 PM_CL="/EHsc /MT /std:c++latest /permissive"
 PM_CL="/EHsc /MT /std:c++latest /permissive- /await"
 PM_CL="/EHsc /MT /std:c++latest /permissive /await"
-PM_CL="/BE /c /EHsc /MD /std:c++latest /permissive- /await"
 PM_CL="/BE /c /EHsc /MD /std:c++latest /permissive-"