Handle "gaps" in C++ vtables. (#7206)

Not every entry in a C++ vtable corresponds to a function that we want
to import. For the holes, leave a `SemIR::InstId::None` in the vtable.
Also mark vtables that extend a C++ vtable as being non-Carbon-native so
we don't try to lower them (and crash on the `None` entries).

In particular, we leave holes for destructors, since we don't have
destructor declarations on the Carbon side that need to override them.

toolchain/check/testdata/interop/cpp/class/import/class.carbon

@@ -522,6 +522,8 @@ fn GenericUse(p2: Generic(Cpp.Class1)) {
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: vtable @Bar.vtable {
+// CHECK:STDOUT:   invalid
+// CHECK:STDOUT:   invalid
 // CHECK:STDOUT:   imports.%Bar.f.decl
 // CHECK:STDOUT: }
 // CHECK:STDOUT: