[win][arm64ec] Handle 'externally available' functions

Author: dpaoliello

llvm/lib/Target/AArch64/AArch64Arm64ECCallLowering.cpp

@@ -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()));
       ThunkMapping.push_back(
           {&F, buildEntryThunk(&F), Arm64ECThunkType::Entry});

llvm/test/CodeGen/AArch64/arm64ec-available-externally.ll

@@ -0,0 +1,9 @@
+; RUN: llc -mtriple arm64ec-windows-msvc < %s
+
+; Arm64EC Regression Test: The Arm64EC Call Lowering was placing "available
+; externally" items in COMDATs, which is not permitted by the module verifier.
+
+define available_externally i32 @f() {
+entry:
+  ret i32 0
+}