Implements fx build tools/fidlgen_llcpp_zircon:validate.
In addition, adds the validation pass to the fuchsia build.

Test: Use fuchsia-mem:llcpp somewhere, then change it. Observe that the
build fails with an error prompting the user to re-run bindings
generation.

Change-Id: I376516c794120fa5faaaf4e2afaffc0283604d09