Export Carbon classes as base / final / abstract. (#7191)

* For Carbon `base class C`, export as a regular C++ class.
* For Carbon `class C`, export with the C++ `final` keyword attribute.
* For Carbon `abstract C`, mark the destructor as pure virtual in cases
where no member function is abstract, or emit an error if the destructor
is not virtual.

To support the final point, mark the destructor of an exported class as
virtual if it overrides a virtual destructor from the base class.

In passing, fix a crash exporting fields if the class has an invalid
base type.

toolchain/check/cpp/import.cpp

@@ -1801,9 +1801,16 @@ static auto ImportFunction(Context& context, SemIR::LocId loc_id,
     }
     if (virtual_modifier != SemIR::Function::VirtualModifier::None) {
       // TODO: Add support for Microsoft/non-Itanium vtables.
+      clang::GlobalDecl decl;
+      if (auto* dtor_decl = dyn_cast<clang::CXXDestructorDecl>(method_decl)) {
+        // TODO: Do we want the index of the complete or base destructor?
+        decl = clang::GlobalDecl(dtor_decl, clang::Dtor_Complete);
+      } else {
+        decl = clang::GlobalDecl(method_decl);
+      }
       virtual_index = dyn_cast<clang::ItaniumVTableContext>(
                           context.ast_context().getVTableContext())
-                          ->getMethodVTableIndex(method_decl);
+                          ->getMethodVTableIndex(decl);
     }
   }