Enable the more effective version of `-Wnon-virtual-dtor`. (#4142)

Most of this is enabled by default, but there is some that needs an
explicit flag.

Avoiding `-Wnon-virtual-dtor` itself for now because that warning can
require changes that carry overhead such as having an extra, unused
destructor entry in the vtable. Hopefully we don't have too many folks
who need our code to be `-Wnon-virtual-dtor` clean.

bazel/cc_toolchains/clang_cc_toolchain_config.bzl

@@ -144,6 +144,7 @@ def _impl(ctx):
                             "-Wextra-semi",
                             "-Wmissing-prototypes",
                             "-Wzero-as-null-pointer-constant",
+                            "-Wdelete-non-virtual-dtor",
                             # Don't warn on external code as we can't
                             # necessarily patch it easily. Note that these have
                             # to be initial directories in the `#include` line.