Import C++ vtables (#7174)

This correctly renders the vtable in SemIR, including allowing overrides
in
Carbon-derived-from-C++ classes.

It doesn't work in lowering because clang walks the methods of the
CXXRecordDecl - and we currently don't export anything into the
CXXRecordDecl's methods (we do export the fields) - so that's next.

This also doesn't teach Clang to affirmatively emit the vtable
regardless of the types use in C++ code - or to have Carbon use the
vtable in an object's initialization.

toolchain/lower/file_context.cpp

@@ -709,6 +709,9 @@ auto FileContext::GetLocForDI(SemIR::InstId inst_id) -> Context::LocForDI {
 auto FileContext::BuildVtable(const SemIR::Vtable& vtable,
                               SemIR::SpecificId specific_id)
     -> llvm::GlobalVariable* {
+  if (!vtable.carbon_native_vtable) {
+    return nullptr;
+  }
   const auto& class_info = sem_ir().classes().Get(vtable.class_id);
 
   SemIR::Mangler m(sem_ir(), context().total_ir_count());