[clang] Reland [C++20] [Modules] [Itanium ABI] Generate the vtable in the mod… (PR #102287)

[Matheus Izvekov via cfe-commits]

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@@ -1124,20 +1124,36 @@ bool Decl::isInAnotherModuleUnit() const {
   if (!M)
     return false;
 
+  // FIXME or NOTE: maybe we need to be clear about the semantics
+  // of clang header modules. e.g., if this lives in a clang header
+  // module included by the current unit, should we return false
+  // here?
+  //
+  // It is clear for header units as the specification says the
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mizvekov wrote:

```suggestion
  // This is clear for header units as the specification says the
```

https://github.com/llvm/llvm-project/pull/102287