Use UB impl of launder on when using clang < 8 and c++17

libstdc++'s definition of std::launder places it behind a check for
__builtin_launder, which is not available before clang 8.

Fixes: #1309

absl/functional/internal/any_invocable.h

@@ -197,7 +197,7 @@ union TypeErasedState {
 template <class T>
 T& ObjectInLocalStorage(TypeErasedState* const state) {
   // We launder here because the storage may be reused with the same type.
-#if ABSL_INTERNAL_CPLUSPLUS_LANG >= 201703L
+#if ABSL_INTERNAL_CPLUSPLUS_LANG >= 201703L && __cpp_lib_launder >= 201606L
   return *std::launder(reinterpret_cast<T*>(&state->storage));
 #elif ABSL_HAVE_BUILTIN(__builtin_launder)
   return *__builtin_launder(reinterpret_cast<T*>(&state->storage));