format_args!() inlining/flattening allows for longer lifetimes

[m-ou-se]

Before #109999, this gave an error:

let f = format_args!("{}", "abc"); // error[E0716]: temporary value dropped while borrowed
dbg!(f);

But after #109999, that no longer gives an error.

Ideally it shouldn't give an error in any case, but that's a separate issue. Inlining/flattening format_args shouldn't change what compiles and what doesn't compile, so, for now, this still shouldn't compile.

[m-ou-se]

Luckily the fix is easy. We just need to make sure that any format_args!(…) with arguments still results in an expansion that involves borrowing a temporary.

[m-ou-se]

Fix here: #110770