Rollup merge of #92828 - Amanieu:unwind-abort, r=dtolnay

Print a helpful message if unwinding aborts when it reaches a nounwind function

This is implemented by routing `TerminatorKind::Abort` back through the panic handler, but with a special flag in the `PanicInfo` which indicates that the panic handler should *not* attempt to unwind the stack and should instead abort immediately.

This is useful for the planned change in https://github.com/rust-lang/lang-team/issues/97 which would make `Drop` impls `nounwind` by default.

### Code

```rust
#![feature(c_unwind)]

fn panic() {
    panic!()
}

extern "C" fn nounwind() {
    panic();
}

fn main() {
    nounwind();
}
```

### Before

```
$ ./test
thread 'main' panicked at 'explicit panic', test.rs:4:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
Illegal instruction (core dumped)
```

### After

```
$ ./test
thread 'main' panicked at 'explicit panic', test.rs:4:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
thread 'main' panicked at 'panic in a function that cannot unwind', test.rs:7:1
stack backtrace:
   0:     0x556f8f86ec9b - <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt::hdccefe11a6ac4396
   1:     0x556f8f88ac6c - core::fmt::write::he152b28c41466ebb
   2:     0x556f8f85d6e2 - std::io::Write::write_fmt::h0c261480ab86f3d3
   3:     0x556f8f8654fa - std::panicking::default_hook::{{closure}}::h5d7346f3ff7f6c1b
   4:     0x556f8f86512b - std::panicking::default_hook::hd85803a1376cac7f
   5:     0x556f8f865a91 - std::panicking::rust_panic_with_hook::h4dc1c5a3036257ac
   6:     0x556f8f86f079 - std::panicking::begin_panic_handler::{{closure}}::hdda1d83c7a9d34d2
   7:     0x556f8f86edc4 - std::sys_common::backtrace::__rust_end_short_backtrace::h5b70ed0cce71e95f
   8:     0x556f8f865592 - rust_begin_unwind
   9:     0x556f8f85a764 - core::panicking::panic_no_unwind::h2606ab3d78c87899
  10:     0x556f8f85b910 - test::nounwind::hade6c7ee65050347
  11:     0x556f8f85b936 - test::main::hdc6e02cb36343525
  12:     0x556f8f85b7e3 - core::ops::function::FnOnce::call_once::h4d02663acfc7597f
  13:     0x556f8f85b739 - std::sys_common::backtrace::__rust_begin_short_backtrace::h071d40135adb0101
  14:     0x556f8f85c149 - std::rt::lang_start::{{closure}}::h70dbfbf38b685e93
  15:     0x556f8f85c791 - std::rt::lang_start_internal::h798f1c0268d525aa
  16:     0x556f8f85c131 - std::rt::lang_start::h476a7ee0a0bb663f
  17:     0x556f8f85b963 - main
  18:     0x7f64c0822b25 - __libc_start_main
  19:     0x556f8f85ae8e - _start
  20:                0x0 - <unknown>
thread panicked while panicking. aborting.
Aborted (core dumped)
```

compiler/rustc_codegen_ssa/src/mir/block.rs

@@ -477,6 +477,28 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
         helper.do_call(self, &mut bx, fn_abi, llfn, &args, None, cleanup);
     }
 
+    fn codegen_abort_terminator(
+        &mut self,
+        helper: TerminatorCodegenHelper<'tcx>,
+        mut bx: Bx,
+        terminator: &mir::Terminator<'tcx>,
+    ) {
+        let span = terminator.source_info.span;
+        self.set_debug_loc(&mut bx, terminator.source_info);
+
+        // Get the location information.
+        let location = self.get_caller_location(&mut bx, terminator.source_info).immediate();
+
+        // Obtain the panic entry point.
+        let def_id = common::langcall(bx.tcx(), Some(span), "", LangItem::PanicNoUnwind);
+        let instance = ty::Instance::mono(bx.tcx(), def_id);
+        let fn_abi = bx.fn_abi_of_instance(instance, ty::List::empty());
+        let llfn = bx.get_fn_addr(instance);
+
+        // Codegen the actual panic invoke/call.
+        helper.do_call(self, &mut bx, fn_abi, llfn, &[location], None, None);
+    }
+
     /// Returns `true` if this is indeed a panic intrinsic and codegen is done.
     fn codegen_panic_intrinsic(
         &mut self,
@@ -1014,10 +1036,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
             mir::TerminatorKind::Resume => self.codegen_resume_terminator(helper, bx),
 
             mir::TerminatorKind::Abort => {
-                bx.abort();
-                // `abort` does not terminate the block, so we still need to generate
-                // an `unreachable` terminator after it.
-                bx.unreachable();
+                self.codegen_abort_terminator(helper, bx, terminator);
             }
 
             mir::TerminatorKind::Goto { target } => {