… Drop implementation

Implement RFC3137 trim-paths sysroot changes - take 2

This PR is a continuation of rust-lang#118149. Nothing really changed, except for rust-lang#129408 which I was able to trigger locally.

Original description:

> Implement parts of rust-lang#111540
>
> Right now, backtraces into sysroot always shows /rustc/$hash in diagnostics, e.g.
>
> ```
> thread 'main' panicked at 'hello world', map-panic.rs:2:50
> stack backtrace:
>    0: std::panicking::begin_panic
>              at /rustc/a55dd71d5fb0ec5a6a3a9e8c27b2127ba491ce52/library/std/src/panicking.rs:616:12
>    1: map_panic::main::{{closure}}
>              at ./map-panic.rs:2:50
>    2: core::option::Option<T>::map
>              at /rustc/a55dd71d5fb0ec5a6a3a9e8c27b2127ba491ce52/library/core/src/option.rs:929:29
>    3: map_panic::main
>              at ./map-panic.rs:2:30
>    4: core::ops::function::FnOnce::call_once
>              at /rustc/a55dd71d5fb0ec5a6a3a9e8c27b2127ba491ce52/library/core/src/ops/function.rs:248:5
> note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
> ```
>
> [RFC 3127 said](https://rust-lang.github.io/rfcs/3127-trim-paths.html#changing-handling-of-sysroot-path-in-rustc)
>
> > We want to change this behaviour such that, when rust-src source files can be discovered, the virtual path is discarded and therefore the local path will be embedded, unless there is a --remap-path-prefix that causes this local path to be remapped in the usual way.
>
> This PR implements this behaviour. When `rust-src` is present at compile time, rustc replaces /rustc/$hash with a real path into local rust-src with best effort. To sanitise this, users must explicitly supply `--remap-path-prefix=<path to rust-src>=foo`.

cc ``@cbeuw``
Fix rust-lang#105907
Fix rust-lang#85463

try-job: dist-x86_64-linux
try-job: x86_64-msvc
try-job: armhf-gnu

…mpiler-errors

Fix diagnostics for coroutines with () as input.

This may be a more real-life example to trigger the diagnostic:

```rust
#![features(try_blocks, coroutine_trait, coroutines)]

use std::ops::Coroutine;

struct Request;
struct Response;
fn get_args() -> Result<String, String> { todo!() }
fn build_request(_arg: String) -> Request { todo!() }
fn work() -> impl Coroutine<Option<Response>, Yield = Request> {
    #[coroutine]
    |_| {
        let r: Result<(), String> = try {
            let req = get_args()?;
            yield build_request(req)
        };
        if let Err(msg) = r {
            eprintln!("Error: {msg}");
        }
    }
}
```

…,fee1-dead

Fix the misleading diagnostic for `let_underscore_drop` on type without `Drop` implementation

Closes: rust-lang#130430
r? rust-lang/diagnostics

Utf8Chunks: add link to Utf8Chunk

It is currently surprisingly non-trivial to go from the `utf8_chunks` method to the docs of the `valid`/`invalid` methods used in the example. This should help.

Update FIXME comment in s390x_unknown_linux_*.rs

- Remove comment about "LLVM < 16" since the minimum external LLVM version is 16+ since rust-lang#117947
- Reflect rename of cabi_s390x.rs in rust-lang@030244c (renamed to [abi/call/s390x.rs](rust-lang@030244c#diff-20136d4a18fa0ef9bd4fc2e6f92e88daad6be88bfb156e5702af39ee87ca4879), and it is currently [still in the same location](https://github.com/rust-lang/rust/blob/9e394f551c050ff03c6fc57f190e0761cf0be6e8/compiler/rustc_target/src/abi/call/s390x.rs)).

r? ```@cuviper```

```@rustbot``` label +O-SystemZ

rustc_target: Add powerpc64 atomic-related features

This adds the following two target features to unstable powerpc_target_feature.

- `partword-atomics`: 8-bit and 16-bit atomic instructions (`l{b,h}arx` and `st{b,h}cx.`) ([definition in LLVM](https://github.com/llvm/llvm-project/blob/llvmorg-19.1.0/llvm/lib/Target/PowerPC/PPC.td#L170-L172))
- `quadword-atomics`: 128-bit atomic instructions (`lqarx` and `stqcx.`) ([definition in LLVM](https://github.com/llvm/llvm-project/blob/llvmorg-19.1.0/llvm/lib/Target/PowerPC/PPC.td#L173-L175))

Both features are [available on power8+](https://github.com/llvm/llvm-project/blob/llvmorg-19.1.0/llvm/lib/Target/PowerPC/PPC.td#L408-L422), so enabled by default for `powerpc64le-*` targets.

r? `@Amanieu`

`@rustbot` label +O-PowerPC