Auto merge of #76570 - cratelyn:implement-rfc-2945-c-unwind-abi, r=Amanieu
Auto merge of #76570 - cratelyn:implement-rfc-2945-c-unwind-abi, r=Amanieu

Implement RFC 2945: "C-unwind" ABI

## Implement RFC 2945: "C-unwind" ABI

This branch implements [RFC 2945]. The tracking issue for this RFC is #74990.

The feature gate for the issue is `#![feature(c_unwind)]`.

This RFC was created as part of the ffi-unwind project group tracked at rust-lang/lang-team#19.

### Changes

Further details will be provided in commit messages, but a high-level overview
of the changes follows:

* A boolean `unwind` payload is added to the `C`, `System`, `Stdcall`,
and `Thiscall` variants, marking whether unwinding across FFI boundaries is
acceptable. The cases where each of these variants' `unwind` member is true
correspond with the `C-unwind`, `system-unwind`, `stdcall-unwind`, and
`thiscall-unwind` ABI strings introduced in RFC 2945 [3].

* This commit adds a `c_unwind` feature gate for the new ABI strings.
Tests for this feature gate are included in `src/test/ui/c-unwind/`, which
ensure that this feature gate works correctly for each of the new ABIs.
A new language features entry in the unstable book is added as well.

* We adjust the `rustc_middle::ty::layout::fn_can_unwind` function,
used to compute whether or not a `FnAbi` object represents a function that
should be able to unwind when `panic=unwind` is in use.

* Changes are also made to
`rustc_mir_build::build::should_abort_on_panic` so that the function ABI is
used to determind whether it should abort, assuming that the `panic=unwind`
strategy is being used, and no explicit unwind attribute was provided.

[RFC 2945]: https://github.com/rust-lang/rfcs/blob/master/text/2945-c-unwind-abi.md

aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust