Fix fnptr not auto implement fn once

[badumbatish]

im a bit rather new to this area...

this fixes #3354

gcc/rust/ChangeLog:

* typecheck/rust-tyty.cc (VariantDef::clone): reformat 
* (VariantDef::monomorphized_clone): reformat 
* (FnPtr::setup_fn_once_output): Fix fnptr not auto implement fn once
* typecheck/rust-tyty.h: Likewise.

gcc/testsuite/ChangeLog:

* rust/compile/format_args_basic_expansion.rs: add sized and fn_once
* rust/compile/issue-2042.rs: add sized and fn_once
* rust/compile/privacy6.rs: add sized and fn_once
* rust/compile/rust_abi.rs: add sized and fn_once
* rust/compile/torture/function_reference2.rs: add sized and fn_once
* rust/compile/torture/function_reference3.rs: add sized and fn_once
* rust/compile/torture/function_reference4.rs: add sized and fn_once
* rust/compile/try-catch-unwind-new.rs: add sized and fn_once
* rust/compile/try-catch-unwind-old.rs: add sized and fn_once

[badumbatish]

i somehow missed privacy6.rs ... will force push soo for merge if Philip doesn't have any change requests

[dkm]

I may be wrong, but I think you're missing the "Signed-off-by" footer (unless you've filed copyright assignment, in that case please ignore my comment) :)