Cannot serialize type kind Opaque with valists and wrap-static-fns

[NickeZ]

Test fails for aarch64-unknown-linux-gnu.

How to reproduce:

$ cargo run -- --wrap-static-fns bindgen-tests/tests/headers/wrap-static-fns.h -- -target aarch64-unknown-linux-gnu
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.08s
     Running `target/debug/bindgen --wrap-static-fns bindgen-tests/tests/headers/wrap-static-fns.h -- -target aarch64-unknown-linux-gnu`
Unable to generate bindings: codegen error: serialization error at bindgen-tests/tests/headers/wrap-static-fns.h:63:56: Cannot serialize type kind Opaque

Was found when trying to enable that target in CI: #3217

There are some more hints in the discussions in that PR.

[miikkas]

(Edit: Had some erroneous findings here previously, replaced with something more useful below.)

I tried minimizing the header file by hand, and the following contents seem to be enough to cause the error:

static void fn1(__builtin_va_list va);

Result	--wrap-static-fns	-- -target aarch64-unknown-linux-gnu
error	x	x
yes	x	-
empty	-	x
yes	-	-