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

[David Blaikie via cfe-commits]

================
@@ -1185,6 +1190,21 @@ bool CodeGenVTables::isVTableExternal(const CXXRecordDecl *RD) {
       TSK == TSK_ExplicitInstantiationDefinition)
     return false;
 
+  // Itanium C++ ABI [5.2.3]:
+  // Virtual tables for dynamic classes are emitted as follows:
+  //
+  // - If the class is templated, the tables are emitted in every object that
+  // references any of them.
+  // - Otherwise, if the class is attached to a module, the tables are uniquely
+  // emitted in the object for the module unit in which it is defined.
+  // - Otherwise, if the class has a key function (see below), the tables are
+  // emitted in the object for the translation unit containing the definition of
+  // the key function. This is unique if the key function is not inline.
+  // - Otherwise, the tables are emitted in every object that references any of
+  // them.
+  if (RD->isInNamedModule())
+    return RD->shouldEmitInExternalSource();
----------------
dwblaikie wrote:

Shouldn't need to test "isInNamedModule" here, though? Would it be adequate to do:
```
if (RD->shouldEmitInExternalSource())
  return true;
```

Ah, I geuss you want to return false ASAP too - I guess looking at some of the implementation details of `shouldEmitInExternalSource` and using those APIs directly here - Always -> true, Never -> false, I think?

Like `adjustGVALinkageForExternalDefinitionKind` already does? Or even reusing some of that functionality somehow.

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