[clang] [CIR] Enable x86_64 calling-convention lowering by default (PR #215026)

[Andy Kaylor via cfe-commits]

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@@ -3770,12 +3770,15 @@ void store_atomic_different_size(S a) {
 
  // FIXME(cir): The difference below is due to ABI lowering not being fully implemented for CIR.
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andykaylor wrote:

This comment is now wrong.

https://github.com/llvm/llvm-project/pull/215026