[llvm] [win][arm64ec] Handle `available_externally` functions (PR #151610)

[Daniel Paoliello via llvm-commits]

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@@ -872,7 +872,7 @@ bool AArch64Arm64ECCallLowering::runOnModule(Module &Mod) {
     if (!F.isDeclaration() && (!F.hasLocalLinkage() || F.hasAddressTaken()) &&
         F.getCallingConv() != CallingConv::ARM64EC_Thunk_Native &&
         F.getCallingConv() != CallingConv::ARM64EC_Thunk_X64) {
-      if (!F.hasComdat())
+      if (!F.hasComdat() && !F.isDeclarationForLinker())
         F.setComdat(Mod.getOrInsertComdat(F.getName()));
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dpaoliello wrote:

Good call: expanded the test and fixed other cases where we should be using `isDeclarationForLinker` instead of `isDeclaration`.

https://github.com/llvm/llvm-project/pull/151610