Test that the built toolchain works with the example Bazel project (#6653)

I wasn't sure I'd be able to really test this code path, but then
I remembered that Bazel has a whole platform for running Bazel from
within an integration test, and it turns out to work brilliantly. It
even lets us point the child Bazel invocations to the just-built
toolchain.

This should both give us confidence that we don't accidentally hit
a Bazel incompatibility with the example project, and it should ensure
that if something about the installed toolchain would stop being
compatible with building via Bazel we'll catch it early.

The tests are integration tests and so a bit slow: 15s or so. But
`//examples/...` is already pretty expensive and no other testing
patterns are impacted.

examples/bazel/example_lib.cpp

@@ -0,0 +1,9 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include "example_lib.h"
+
+#include <iostream>
+
+auto HelloWorld() -> void { std::cout << "Hello World!\n"; }