Verify that trans::native::build_direct_fn gets ABIs right

[catamorphism]

There's a FIXME suggesting that it may not.

[emberian]

Renominating for backwards compatible.

[graydon]

backwards compat is about source code compatibility, not linkage.

[graydon]

accepted for well-covered milestone

[sanxiyn]

I can't find the FIXME. Does anyone know what this was about?

[klutzy]

the function was removed by #8535.