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

[Chuanqi Xu via cfe-commits]

================
@@ -1180,6 +1185,21 @@ 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 (Module *M = RD->getOwningModule(); M && M->isNamedModule())
----------------
ChuanqiXu9 wrote:

> hasExternalDefinitions isn't "is this AST from some other AST file" but "did the AST file say it'd handle the object-file definition of this thing" - the pcm file specifies which things it'll handle.
> So if the intent is not to home anything in the GMF, then pcms could be written out that don't set "hasExternalDefinitions" to true for those entities.

No, it is not the intent. The intent is to keep GMF as legacy code instead of named modules.

For example,

```
// header.h
void func() { } // ! Non inline

// x.cppm
module;
#include "header.h"
export module x;
...
```

Then in the object file of `x.cppm`, (if `func()` is not discarded out), we should be able to see the definition of `func()`. Although it is rare since the definitions in the headers are generally inline, but we as the compiler can't assume that.

I think the current implementation is more straight forward and clear.

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