Auto merge of #106824 - m-ou-se:format-args-flatten, r=oli-obk

Flatten/inline format_args!() and (string and int) literal arguments into format_args!()

Implements https://github.com/rust-lang/rust/issues/78356

Gated behind `-Zflatten-format-args=yes`.

Part of #99012

This change inlines string literals, integer literals and nested format_args!() into format_args!() during ast lowering, making all of the following pairs result in equivalent hir:

```rust
println!("Hello, {}!", "World");
println!("Hello, World!");
```

```rust
println!("[info] {}", format_args!("error"));
println!("[info] error");
```

```rust
println!("[{}] {}", status, format_args!("error: {}", msg));
println!("[{}] error: {}", status, msg);
```

```rust
println!("{} + {} = {}", 1, 2, 1 + 2);
println!("1 + 2 = {}", 1 + 2);
```

And so on.

This is useful for macros. E.g. a `log::info!()` macro could just pass the tokens from the user directly into a `format_args!()` that gets efficiently flattened/inlined into a `format_args!("info: {}")`.

It also means that `dbg!(x)` will have its file, line, and expression name inlined:

```rust
eprintln!("[{}:{}] {} = {:#?}", file!(), line!(), stringify!(x), x); // before
eprintln!("[example.rs:1] x = {:#?}", x); // after
```

Which can be nice in some cases, but also means a lot more unique static strings than before if dbg!() is used a lot.

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