[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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@@ -41,9 +43,10 @@ Base::~Base() {}
 // CHECK: @_ZTSW3Mod4Base = constant
 // CHECK: @_ZTIW3Mod4Base = constant
 
-// CHECK-INLINE: @_ZTVW3Mod4Base = linkonce_odr {{.*}}unnamed_addr constant
-// CHECK-INLINE: @_ZTSW3Mod4Base = linkonce_odr {{.*}}constant
-// CHECK-INLINE: @_ZTIW3Mod4Base = linkonce_odr {{.*}}constant
+// With the new Itanium C++ ABI, the linkage of vtables in modules don't need to be linkonce ODR.
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dwblaikie wrote:

Would they benefit from still being linkonce? What if the vtable is never used? Or do we leave that up to `--gc-sections` linker behavior? (I'm not clear on when we use linkage to let things be dropped, and when we use the linker function-sections/data-sections/gc-sections behavior to drop unused stuff)

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