[clang] [llvm] [SPIR-V] Enable variadic function lowering for the SPIR-V target (PR #175076)

[Matt Arsenault via cfe-commits]

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@@ -249,7 +250,8 @@ class ExpandVariadics : public ModulePass {
         return false;
       }
 
-      if (CI->getCallingConv() != CallingConv::C)
+      if (CI->getCallingConv() != CallingConv::C &&
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arsenm wrote:

Given we rely on this for lowering, that's a bug 

https://github.com/llvm/llvm-project/pull/175076