Fix the Apple LLVM version when disabling tests for Xcode 12

Summary: [Folly] Fix the Apple LLVM version when disabling is-constexpr-default-constructible test expectations over types with non-trivial destructors for Xcode 12.

Differential Revision: D23917973

fbshipit-source-id: 1238ec5a53ba0ff73afd67f81ad2868f37196eaa

folly/Portability.h

@@ -454,6 +454,12 @@ constexpr auto kIsWindows = true;
 constexpr auto kIsWindows = false;
 #endif
 
+#if defined(__APPLE__)
+constexpr auto kIsApple = true;
+#else
+constexpr auto kIsApple = false;
+#endif
+
 constexpr bool kIsAppleIOS = FOLLY_APPLE_IOS == 1;
 constexpr bool kIsAppleMacOS = FOLLY_APPLE_MACOS == 1;
 constexpr bool kIsAppleTVOS = FOLLY_APPLE_TVOS == 1;

folly/test/TraitsTest.cpp

@@ -436,7 +436,7 @@ TEST(Traits, is_constexpr_default_constructible) {
   // under clang 10, crash: https://bugs.llvm.org/show_bug.cgi?id=47620
   // and, with assertions disabled, expectation failures showing compiler
   // deviation from the language spec
-  if (kClangVerMajor != 10) {
+  if (kClangVerMajor != (kIsApple ? 12 : 10)) {
     struct NonTrivialDtor {
       ~NonTrivialDtor() {}
     };