[clang] [Arm64EC][clang] Implement varargs support in clang. (PR #152411)

[Eli Friedman via cfe-commits]

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@@ -1409,6 +1409,12 @@ class WinX86_64ABIInfo : public ABIInfo {
     return isX86VectorCallAggregateSmallEnough(NumMembers);
   }
 
+  ABIArgInfo classifyArgForArm64ECVarArg(QualType Ty) const override {
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efriedma-quic wrote:

classifyArgForArm64ECVarArg is exclusively for use by Arm64EC calling convention code... but it's implemented in the x64 calling convention code because it's exactly the algorithm we've already implemented for x64, and I don't want to copy-paste it.

I could refactor the code further to extract the underlying algorithm out of WinX86_64ABIInfo, but that doesn't seem like it would make the code more readable overall.

https://github.com/llvm/llvm-project/pull/152411