[clang] [C++20] [Modules] [Itanium ABI] Generate the vtable in the module unit of dynamic classes (PR #75912)

[David Blaikie via cfe-commits]

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@@ -1802,6 +1802,12 @@ void ItaniumCXXABI::emitVTableDefinitions(CodeGenVTables &CGVT,
   if (VTable->hasInitializer())
     return;
 
+  // If the class is attached to a C++ named module other than the one
+  // we're currently compiling, the vtable should be defined there.
+  if (Module *M = RD->getOwningModule();
+      M && M->isNamedModule() && M != CGM.getContext().getCurrentNamedModule())
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dwblaikie wrote:

this idiom is repeated in a few places, and is a bit long-winded. Perhaps it could be made into some common function/shared (if the `hasExternalDefinition` thing can't be made to work more generally)

https://github.com/llvm/llvm-project/pull/75912