[clang] [X86_64] fix empty structure vaarg in c++ (PR #77907)

[Phoebe Wang via cfe-commits]

================
@@ -2989,6 +2989,11 @@ static Address EmitX86_64VAArgFromMemory(CodeGenFunction &CGF,
   // an 8 byte boundary.
 
   uint64_t SizeInBytes = (CGF.getContext().getTypeSize(Ty) + 7) / 8;
+
+  if (isEmptyRecord(CGF.getContext(), Ty, true)) {
+    SizeInBytes = 0;
+  }
+
----------------
phoebewang wrote:

Suggestion:
```
uint64_t SizeInBytes = 0;
if (!isEmptyRecord(CGF.getContext(), Ty, true))
  SizeInBytes = (CGF.getContext().getTypeSize(Ty) + 7) / 8;
```

https://github.com/llvm/llvm-project/pull/77907