Export abstract methods as pure virtual (#7578)

`abstract fn` was exported to C++ as a plain virtual function rather
than a pure virtual one, so the class wasn't abstract and could be
instantiated from C++.

Abstract functions no longer get a thunk since there is no definition to
call. The tests are prefixed with `fail_` since an abstract class still
errors on `Core.Destroy` regardless.

toolchain/check/cpp/generate_ast.cpp

@@ -544,6 +544,11 @@ auto CarbonExternalASTSource::CompleteType(clang::TagDecl* tag_decl) -> void {
                method_decl,
                MakeVirtualFunctionSignature(*context_, method_decl)),
            .inst_id = function.first_decl_id()});
+      // An abstract function has no definition, so it doesn't need a thunk.
+      if (function.virtual_modifier ==
+          SemIR::Function::VirtualModifier::Abstract) {
+        continue;
+      }
       pending_virtual_functions.push_back(
           {.loc_id = SemIR::LocId(vtable_entry_id),
            .function_id = callee_function.function_id,