Add a method to get the `FnAbi` of a function pointer

[celinval]

We have added a method to get the function ABI of an instance, however, that doesn't work for function pointers. We should consider adding a similar API to the query fn_abi_of_fn_ptr(/*..*/).

[oli-obk]

function pointer types have a fn sig field https://doc.rust-lang.org/nightly/nightly-rustc/stable_mir/ty/struct.FnSig.html that contains the ABI.

[celinval]

That just represents the ABI type not the argument's ABI though. I changed the title of this issue to avoid any confusion. Sorry about that!

[adwinwhite]

@rustbot claim