[clang] [llvm] [LLVMABI][AARCH64] Handle condtions for ignoring empty types (PR #224476)

[Peter Smith via cfe-commits]

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@@ -144,12 +148,25 @@ ArgInfo AArch64TargetInfo::classifyArgumentType(
                                            RecordArgABI::RAA_DirectInMemory);
   }
 
+  // Empty records:
+  // AAPCS64 does not say that empty records are ignored as arguments,
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smithp35 wrote:

That's possibly something we can improve in the ABI, feel free to raise issues in 
https://github.com/ARM-software/abi-aa/issues

My understanding is that this a GCC extension https://gcc.gnu.org/onlinedocs/gcc-4.1.2/gcc/Empty-Structures.html which maybe why it isn't in the base ABI.

The ABI is clear on C++ https://github.com/ARM-software/abi-aa/blob/main/cppabi64/cppabi64.rst#41summary-of-differences-from-and-additions-to-the-generic-c-abi 

https://github.com/llvm/llvm-project/pull/224476