[clang] fix: compatible C++ empty record with align UB with gcc (PR #72197)

[via cfe-commits]

================
@@ -307,7 +307,12 @@ AArch64ABIInfo::classifyArgumentType(QualType Ty, bool IsVariadic,
     // 0.
     if (IsEmpty && Size == 0)
       return ABIArgInfo::getIgnore();
-    return ABIArgInfo::getDirect(llvm::Type::getInt8Ty(getVMContext()));
+    // An empty struct can have size greater than one byte if alignment is
+    // involved.
+    // When size <= 64, we still hold it by i8 in IR and lowering to registers.
+    // When Size > 64, just fall through to avoid va_list out of sync.
----------------
hstk30-hw wrote:

Forgive my foolish，can I just copy it as comment?  

> AAPCS64 does not say that empty records are ignored as arguments,
> but other compilers do so in certain situations, and we copy that behavior.
> Those situations are in fact language-mode-specific, which seems really
> unfortunate, but it's something we just have to accept. If this doesn't
> apply, just fall through to the standard argument-handling path.
> Darwin overrides the psABI here to ignore all empty records in all modes.


https://github.com/llvm/llvm-project/pull/72197