rust: rust_is_available: warn for `bindgen` < 0.72.1 && libclang >= 22

Starting with LLVM 22, `clang_getTypeDeclaration()` may return a forward
declaration instead of the type definition. This made `bindgen` generate
opaque types [1][2], which in turn made us fail with e.g.

    error[E0609]: no field `__bindgen_anon_1` on type `bindings::kernel_param`
      --> rust/kernel/module_param.rs:78:46
       |
    78 |         let container = unsafe { &*((*param).__bindgen_anon_1.arg.cast::<SetOnce<T>>()) };
       |                                              ^^^^^^^^^^^^^^^^ unknown field
       |
       = note: available field is: `_address`

This was fixed in `bindgen` 0.72.1 [3].

In order to clarify what is going on and avoid confusion [4][5], add
a warning to `rust_is_available.sh` about it when the versions match,
similar to past warnings like the one removed in:

  commit ae64324ad5c1 ("rust: rust_is_available: remove warning for `bindgen` < 0.69.5 && libclang >= 19.1")

In addition, even if the versions match, check if the issue appears to
not reproduce with the given binaries, to avoid a warning in such a case.

Finally, include tests.

[ Nathan, in parallel, updated the instructions of the LLVM+Rust
  kernel.org toolchains [6] so that `--version` is not passed to
  `cargo` for `bindgen`, and thus the latest `bindgen` is installed
  by default, which should help to avoid some of these situations.

  Thanks!

    - Miguel ]

Cc: stable@vger.kernel.org # Needed in 6.12.y and later (Rust is pinned in older LTSs).
Link: https://github.com/rust-lang/rust-bindgen/issues/3264 [1]
Link: https://github.com/Rust-for-Linux/linux/issues/353 [2] # "Missing fields in nested class with LLVM 22."
Link: https://github.com/rust-lang/rust-bindgen/pull/3278 [3]
Reported-by: Burak Emir <burak.emir@gmail.com>
Link: https://github.com/Rust-for-Linux/linux/issues/1247 [4]
Link: https://lore.kernel.org/rust-for-linux/CABwQupNfMAJOGqRM9ke6tj4f53dCCsBDKU7Vp+zf8mwk7bqt8Q@mail.gmail.com/ [5]
Link: https://mirrors.edge.kernel.org/pub/tools/llvm/rust/ [6]
Tested-by: Burak Emir <burak.emir@gmail.com>
Link: https://patch.msgid.link/20260719120514.159914-1-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

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