[clang] [ARM] Empty structs are 1-byte for C++ ABI (PR #124762)

[Eli Friedman via cfe-commits]

================
@@ -366,9 +387,15 @@ ABIArgInfo ARMABIInfo::classifyArgumentType(QualType Ty, bool isVariadic,
     return getNaturalAlignIndirect(Ty, RAA == CGCXXABI::RAA_DirectInMemory);
   }
 
-  // Ignore empty records.
-  if (isEmptyRecord(getContext(), Ty, true))
-    return ABIArgInfo::getIgnore();
+  // Empty records are either ignored completely or passed as if they were a
+  // 1-byte object, depending on the ABI and language standard.
+  if (isEmptyRecord(getContext(), Ty, true) ||
+      getContext().getTypeSize(Ty) == 0) {
+    if (shouldIgnoreEmptyArg(Ty))
----------------
efriedma-quic wrote:

I think I'd prefer to write this something like:

```
if (size == zero)
  return getIgnore();
if (isempty() && isWatchOS())
  return getIgnore();
```

This should be equivalent because an empty struct in C is size zero anyway.

https://github.com/llvm/llvm-project/pull/124762