Add a predicate script for determining whether `bindgen` can process a given header

First, `bindgen` is run on the input header. Then the emitted bindings are
compiled with `rustc`. Finally, the compiled bindings' layout tests are run.

By default, this script will exit zero if all of the above steps are successful,
and non-zero if any of them fail. This is appropriate for determining if some
test case (perhaps generated with `csmith` or another fuzzer) uncovers any bugs
in `bindgen`.

However, this script can also be used when reducing (perhaps with `creduce`) a
known-bad test case into a new, smaller test case that exhibits the same bad
behavior. In this mode, you might expect that the emitted bindings fail to
compile with `rustc`, and want to exit non-zero early if that is not the
case. See the "reducing arguments" section for details and what knobs are
available.

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