[clang] [llvm] [CodeGen][i386] Move -mregparm storage earlier and fix Runtime calls (PR #89707)

[Eli Friedman via cfe-commits]

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@@ -4781,6 +4782,7 @@ CodeGenModule::CreateRuntimeFunction(llvm::FunctionType *FTy, StringRef Name,
         }
       }
       setDSOLocal(F);
+      markRegisterParameterAttributes(F);
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efriedma-quic wrote:

To clarify, the code in llvm/ still needs to exist because we need some approximation of the calling convention even if we don't have access to the clang AST.  But clang shouldn't use it; clang's native representation of calling conventions is better.

(It's a long-standing request to try to give LLVM a representation of calling conventions that's closer to clang's representation, but it's a large problem, and nobody has really made any progress on it.)

https://github.com/llvm/llvm-project/pull/89707