[libcxx] [test] In msvc_stdlib_force_include.hpp, use _HAS_CXX17 to set TEST_STD_VER.

_HAS_CXX17 indicates whether MSVC's STL is in C++17 mode.

In MSVC there's a distinction between CRT headers like stdlib.h and STL headers
like cstdlib. Only the STL headers drag in yvals.h, our internal STL-wide header
that defines internal macros like _HAS_CXX17.

_HAS_CXX17 is an MSVC STL library macro, unconditionally defined. We centralize
everything on this, because we have to ask different questions to determine
whether C1XX, EDG, or Clang is in 14 or 17 mode, and we additionally permit
users to override the detection in one way (it's okay to ask for 17 from the
compiler, but only 14 from the libs, at least for the moment; only noexcept
in the type system will give us a headache).

As this header is for testing MSVC's STL, we can assume _HAS_CXX17 is defined.

Fixes D32726.

llvm-svn: 302104

libcxx/test/support/msvc_stdlib_force_include.hpp

@@ -40,8 +40,6 @@ const AssertionDialogAvoider assertion_dialog_avoider{};
 
 // MSVC frontend only configurations
 #if !defined(__clang__)
-    #define TEST_STD_VER 17
-
     // Simulate feature-test macros.
     #define __has_feature(X) _MSVC_HAS_FEATURE_ ## X
     #define _MSVC_HAS_FEATURE_cxx_exceptions    1
@@ -75,4 +73,12 @@ const AssertionDialogAvoider assertion_dialog_avoider{};
 // Silence warnings about raw pointers and other unchecked iterators.
 #define _SCL_SECURE_NO_WARNINGS
 
+#include <ciso646>
+
+#if _HAS_CXX17
+    #define TEST_STD_VER 17
+#else // _HAS_CXX17
+    #define TEST_STD_VER 14
+#endif // _HAS_CXX17
+
 #endif // SUPPORT_MSVC_STDLIB_FORCE_INCLUDE_HPP