[libcxx] [test] Clarify weak_ptr_ret on Windows, remove a LIBCXX-WINDOWS-FIXME

[mstorsjo]

Summary

On Windows, structs with a destructor are always returned indirectly;
add this to the list of known exceptions in the test where the class
isn't returned in registers as expected.

[mstorsjo]

Rebase past CI config changes

libcxx/test/libcxx/memory/trivial_abi/weak_ptr_ret.pass.cpp

 
 // Test weak_ptr<T> with trivial_abi as return-type.
 
 // ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ABI_ENABLE_SHARED_PTR_TRIVIAL_ABI
 
 // There were assertion failures in both parse and codegen, which are fixed in clang 11.
 // UNSUPPORTED: gcc, clang-4, clang-5, clang-6, clang-7, clang-8, clang-9, clang-10
 
-// XFAIL: LIBCXX-WINDOWS-FIXME
-
 #include <memory>
 #include <cassert>
 
 __attribute__((noinline)) void call_something() { asm volatile(""); }
 
 struct Node {
   explicit Node() {}
   ~Node() {}
int main(int, char**) {
   std::weak_ptr<Node> ret = make_val(sptr, &local_addr);
   assert(local_addr != nullptr);
 
   // Without trivial_abi, &ret == local_addr because the return value
   // is allocated here in main's stackframe.
   //
   // With trivial_abi, local_addr is the address of a local variable in
   // make_val, and hence different from &ret.
-#if !defined(__i386__) && !defined(__arm__)
+#if !defined(__i386__) && !defined(__arm__) && !defined(_WIN32)
   // On X86, structs are never returned in registers.
   // On ARM32, structs larger than 4 bytes cannot be returned in registers.
+  // On Windows, structs with a destructor are always returned indirectly.
   // Thus, weak_ptr will be passed indirectly even if it is trivial.
   assert((void*)&ret != local_addr);
 #endif
   return 0;
 }